I may be wrong, but I'm fairly confident it's mostly preference. Shorter cranks are going to be better for spinning, but you won't have as much leverage since the arms aren't as long. Longer cranks are going to have more leverage and thus easier to get going but maybe a bit more awkward to be spinning since there will be a larger range of motion. On some bikes(like fixed gears, or bikes with particularly low bottom brackets), it's more crucial to select the proper crank length to avoid pedal strike on turns, but I believe that's not as much of an issue on modern road bikes.