If you were running longer than 200m distance, your shoes should be providing a significant gap between your toes and the end of the shoe. Probably 400 to 1500m runners still look for precision in their track spikes so a 5mm to 1cm of functional space is enough, but for anything else, larger shoes are necessary.
I always recommend the 1.5cm rule, what is most often the width of your big thumb. That works really fine for healthy individuals. I say healthy, as if you have serious salt retention and bodily inflammation, your feet can swell up multiple sizes even during a relatively short ultra marathon of 6hours. I feel that till 24hours, if we were healthy, the 1.5cm rule works extremely fine. Over that like during multi-day hike-run adventures, 200milers, or 6day events, probably 2cm to 2.5cm will be needed.
There can be modifications in case of little feet or giant feet. So, if your sizing is under 40EU, you might consider 1 to 1.2cm of gap. For 40 to 46EU the 1.5cm applies and over 46 probably better go with 2cm gap. Little girls of 45 to 50kg with little feet running mostly shorter distances might even go with 0.5 to 0.8cm toe gap and will not ever have any black toenails or blisters ! Shoe sizing is individual and some trial and error will be needed at the beginning of anyone's running career.
When I see people choosing shoes over the 2.5cm, that is most likely due to their experience with that particular shoe. Often it is not really well understood, that the problem comes from the width of the shoe, the drop of the shoe and the overall shape of the shoe. The shape of the heel and heel collar, the midfoot and forefoot area, but also the features of the upper.
In 99% of all cases, including Marathon des Sables, correct dietary habits and correct shoe shape make it unnecessary to choose a shoe longer than your feet + 2.5cm.
Your running form also impacts the shoe you should choose. It should be actually the opposite, but people don't listen and don't even want to listen. What I am saying here, is that every single person interested about running, should be focusing on great biomechanics and properly balanced forefoot stride, while maintaining proper posture and breathing. Choosing shoes what are flexible, light, feet shaped, flat should be priority.
Just because something doesn't hurt initially, it can still be a silent killer ! Just like smoking. If you had an incorrect stride, your highly cushioned or/and highly structured shoe might protect your from immediate pain, but won't help in case of strength, form, stride and can result in an overuse injury coming from nowhere and never going away till the rest of your life, period ! We have seen people running holes on their knees, having permanent achilles calcification, destroyed hip-sockets and exercise slash running induced spinal degeneration !
We can see this coming true in Hoka One One shoes. You are a great runner ? Just by adding a couple of millimetres of foam, but staying light on your feet, will change nothing. You are a not so great runner ? The foam will collapse on the side where your stride comes short and it will even further accentuate your initial issue. The same true to structured shoes. When a double density foam is inserted on the inside for pronators or on the outside for supinators / overpronators, instead of strengthening you, while working against bad habits, will let you have your bad habits, till the foam wears off. This is true to overbuilt heelcounters, arch-wraps, insoles, transversal plastic inserts, discs and so. Actually this is why Mizuno is in trouble, because all of their shoes are based on a heavy, heel striking model, having their most important Wave tech under the heel. When we are forefoot running it is unnecessary and if placed under the forefoot it feels horrible.
Being said all of this, if used strategically, inserts might still help ! For instance the racing shoe Fastwitch from Saucony, is still a light platform with 4mm drop. However it has a built up inner heel for protecting you caving your knees in. Runners having some knee issues just before an important race can pull this feature out from the magic hat once in a while and go through the event, with no further damage or injuries.
It is not a final, decision making argument-point, but can be one of the steps in the right direction.
There can be some compromises made, sometimes. In the last couple of years, new tech arose in case of materials used. One is the knit type mesh. We can see them all over the place. In Altra, Saucony, Nike, Adidas and so. The benefits are expandability without stretchiness. It kind of lets your feet breathe, but won't give you a sloppy, too large of a shoe feeling. This can allow you to wear some shoes, what were out of the picture for a while !
My foot shape doesn't correspond at all to the Saucony Type A shoes. At all ! I have big square shaped feet, wide forefoot, toes lined up straight. However because of performance reasons, I keep coming back to this shoe every year. This latest model with the knit type upper is the most comfortable since the Type A4 ! Ever ! I already put in a podium performance in an urban trail race, out of the box, with no break in ! No blisters, no hotspots and no problems ! The foam, the bendiness, the lack of heel counter, the upper, the 4mil drop and so make this shoe ideal for my running style. Using mostly Altra and the Merrell Bare line in training, this is a very different shoe.
There is also the performance argument. You can see in case of sprinters, speed skaters, speed skiers, time trial and track cyclists, that they deliberately choose the lightest, stiffest, more closest fitting and bearable shoes possible. Why ? If you had a 1000th of a millimetre of a move in a shoe, that is called a power leak ! In case of sprinting 100 meters it can mean the difference between 9.58 and 9.579. In case of an hour of cycling on the track, that might show up in 5second between you and the world record ! For a 5k or a 10k, I find that a correct length, but more likely close fitting shoe is the way to go. You still don't want to jam your toes together and have your big toe pushed inwards like in a Manolo Blahnik. Definitely not having your toes smashed to the front of the shoe !
IN THE SHOP
First, do not hesitate to go on shoe tests. These occasions are very beneficial ! You can try on road and trail running shoes, maybe even exceeding 30minutes. It might help you discover new brands corresponding to your running style !
Prepare yourself. Nails clipped and filed. Have your own socks with you. Do not buy a pair of new socks in the shop ! The shoe will feel fantastic even if it was not. New soft socks in a fresh new pair of shoes ? Business !
If you were a "pro" or want to act like one and not really sure about your size, try the shoes on early morning. Go back after a full day in the office and try it on again. The difference can be shocking. Feet and ankles swell during the day. This can represent a swelling in a marathon or an ultramarathon.
First take out the insole. Place your naked feet on it. Both of them. This should already correspond to your foot shape and measuring the 1 to 1.5cm gap at the end. Do the same with socks on. It should still be correct. No toes going over the edge please ! If your forefoot was over like 2mm, that is not a big issue, but if the end of your toes overlap the edge of the insole, that is problematic.
Using your own insoles can be problematic. Shoe depth in all parts of the shoe comes into the picture. It is another subject and won't discuss it here. My view on orthopaedic insoles is radical. They are great for pain relief and for correcting improper biomechanics. They provide correct posture and form for walking and running.
They are help ! However, would you rely on help for your entire life ? When breaking an ankle, are you in cast for the rest of your life ? When burning your hand, do you leave the plaster on forever ? No !
Medical insoles are part of rehabilitation and can be incorporated time to time in exercise routines and the daily life during the recovery period. If you had jammed toes, bunions, collapsed arches and so, yes, this can take as long as 5 years or more and maybe some surgeries will be necessary too. Breaking down calcified and deformed tissue and reforming joints, bones, tendons and ligaments, reeducating muscles and nerves are a difficult, but highly rewarding project. This can mean some suffering and a decline in performance, but a brutal increase in quality of life, especially in older ages ! Indirectly foot shape effects breathing, digestion, blood flow, nerve health and much more. Diving into the woo-woo of energy lines, acupressure points, chi and so, all comes from the way you are grounded.
The shoe store should have a treadmill or a small running circuit installed. You should be able to do some strides in your chosen model. This will allow you to determine the drop and the flexibility for your needs.
I know that my ideal drop is 0 to 4mm and if it is over I feel it immediately. I also feel false data. Putting on a 4mm shoe, but actually it feels more likely a 6 or and 8mm shoe. This can be due to real differences or an inappropriately designed incline. Too sudden of a drop in the middle of the shoe instead of a gradual ramp. For instance the Brooks Cascadia shoe. It has a 1cm drop, but on a so gradual platform that it feels more like a 6mm one. The Bushido from La Sportiva has only 6mm drop, but the thin heel and the steep arch make it feel way more and I always twist my ankles in that shoe. It is sent to Africa !
Flexibility can be chosen by the destiny of the shoe. The more flexible a trail shoe is, the more exposed some spots will be for rock bruises. The less flexible it is, the more it will modify your stride and can result in overuse injuries. The same true to road shoes. A carbon propulsion plate might aid you in an out and back 10k, but keep you from effectively handling sharp switchbacks, up and downhill running.
Your foot function will also determine the current flexibility needs. A 4mm drop Nike Free 3.0 can be a great beginner's practice shoe, as it has cushioning, flexibility and some stability to it, however I wouldn't go cold turkey into a Vivobarefoot or Skora. Your progression and decisions are up to you. Scrambling and road running are two different things.
The lacing and the upper are also dependent on the foot shape you have. I have high instep and a bony top of the foot. I love tortilla wrap style tongue with a slightly asymmetric lacing system. The Nike Kiger 1 and the old Brooks Green Silence were lovely models for me. You might be a straight down guy or a BOA person. Maybe using those triathlete elastic quick laces.
Road, gravel, sandy beach rode, trail, mountain trail, groomed trail, scrambling and more. On a bad quality road surface with some dust on it, a simple EvA exposed midsole like we can see in many all out racing models is a bad idea. You won't have any grip for the push off. A soft rubber might have extra ordinary grip for scrambling and slower movements, but would file down when bombing fast downhill. I like Vibram, but it can too much rigidify a shoe. That can be also a good thing though, like in the Altra King MT. Choose your material wisely.
Deep lugs are necessary for mud and snow. However as they wear off on hard packed terrain, they can be a stride modifying feature, potentially causing an overuse injury.
There is no one correct grip. Different conditions require different lug patterns, length and materials.
Trying on the shoe you need, at the afternoon is your best bet. Forget about sizing. Get out the insole, place your feet on it and hit the 1.5cm gap, with no overlapping toes. Choose a shoe with a drop ideal for your current running style. If your goal is health, movement, speed, proper feet function, performance, posture and more, work towards a low drop, light and wide toe-box flexible shoe. It is a long, gradual process.
I find in the last 10 years working with customers, that UK sizing is the most consistent. US is okay, but European sizing are all over the place. I have shoes from 46 to 49EU. All of them are UK12 or 12.5 though ! There are shoes like Salomon, Hoka, Adidas and so with the 1/3 system. Very often if not always, you have to size up from your regular EU size as they don't correspond to the UK size. So I use mostly UK12 what comes down to 48, but in those brands it is UK12.5 to go up to 48, as the UK12 & 47 1/3 is way too small.
American brands like Nike and Brooks go with UK12 and 47.5 what are also 31cm long.
Most likely in case of winter shoes made of GoreTex, you must size up. The materials are rigid, you will use thicker socks, but they are also less flexible.
It is a mess, so don't bother. Get the insole out and place your feet on it.