There is a range of usable crank lenghts for any rider (probably +- 1cm) but the size should be proportional in some way to leg length. This is more of a problem for big or small riders; medium riders can ride most cranks without discomfort.
Crank length interacts with frame and wheel size: the BB height should place the pedals to avoid grounding on corners so shorter cranks can have lower BB height (and visa versa for long cranks). This makes mount/dismount much easier for small riders. The BB height on most production bikes varies very little across the size range.
Cranks can interfere with the front wheel so designers should match the wheel size and length of the bike accordingly.