there is no need to have a specific gallon. The best way to figure it out is to determine what and how your going to use your sump. If its just for equipment, then you figure out what your gonna be using and find a tank that can fit it all. If ur adding a refugium, then u'll obviously need a larger space. Of course you are limited by the space under your stand as well.
It depends what you want in the sump. Are you going to have a fuge? It also depends how much room you have to spare, internal vs. external pump, etc. I don't think there is any minimum or maximum size for a sump.