Rock shoe anatomy
Why is fit important?
If your shoe doesn't map your foot precisely and there are gaps between the two, when you stand on a small hold your foot may roll in the shoe causing it to slip off the hold. However, overly tight rock shoes can ruin the best route in the world with excruciating pain.
How to fit a rock shoe
The softer the sole the tighter the shoe must be. A very stiff sole will act like a piece of wood strapped under your foot.
Find a model that fits the shape of your foot. Most people climb with no socks so that you get more 'feeling', so take off your socks when you try the shoes on. A perfect fitting rock shoe will fit like a rubber sock without any spaces around your toes or heel. The laces need to be at least 15mm apart so that they can be tightened up as the shoes stretches.
How small do they need to be?
This depends greatly on how long you want to wear them for. A boulderer or sport climber will need maximum performance and therefore a very close fitting shoe. Some elite climbers use the very smallest shoe that they can physically get on their feet, perhaps three sizes smaller than their normal shoe size! In contrast, for something like a 500ft VS traditional route, comfort is a much higher priority so sizing is not as critical.
The softer the sole the tighter the shoe must be. A very stiff sole will act like a piece of wood strapped under your foot. This can give you amazing support. Climbers who have comfort high on their priority list will normally opt for a stiff sole and size generously.
How much will they stretch?
This is governed by the materials, and whether or not the shoe has a lining. A soft leather slipper without a lining can stretch dramatically, sometimes two British sizes. Linings reduce the stretch and a fully lined shoe will barely stretch at all. With new technology coming through some manufactures are starting to use elastic liners to work actively with your feet.
Symmetrical or Asymmetrical?
Everyone's feet are different and the day that someone produces a shoe that fits everyone well, will go down in climbing history. Symmetrical shoes have the front toe point in the middle. This is an old, very general design, but it still fits people with long middle toes and short big toes! Rock shoes are now becoming more asymmetrical where the shape follows the anatomy of the foot and the front point is over the big toe. If your big toe is the longest this is what you need.
Very high performance shoes and slippers may have extreme asymmetry where the big toe is turned towards the inside of the foot. If you add some 'cambering' to this you will end up with something totally radical and superb for extreme climbing on steep rock.
If you put a rock shoe on a flat table and the sole is parallel to it, the shoe has no cambering. The more the sole is concave, the more cambering it has. Cambering is excellent for steep rock allowing you to use your feet like hands. Curling your toes down and pulling in with your feet stops your feet 'popping off' marginal holds. Cambering also makes a massive difference for high steps and rockovers where you can use your feet a lot more actively.