It's not only a hand size and scale length thing. A lot has to do with the way your hand is shaped when bent, the directions your fingers point when you make a fretting posture with your hand, the width of the neck, the depth of the neck, the shape of the neck, etc. Some small-handed people can play certain long-scale basses easily. I have longer hands (I am 6'3") and although i can play long scale, I have a much easier time with short scale because when I flex my fingers around a neck, my fingers all tend to point toward a single spot, instead of being more parallel to one another. It's just physiology. No amount of practice will change the way my fingers point, the only option I would have would be the wearing of some orthopedic device all the time to try and force my fingers to be more parallel when in a fretting pose in the hopes that my hand would become trained that way.
