Rob Id go with something around 15-20g for the sump. There is no rule on how big the sump should be. The key is to be able to fit all the equipment you want to fit in the sump (Skimmer, pump, heater etc....) You may not have room for a auto top off so you will have to be creative on that one. You will have a cycle, most likely a small cycle but you will have one.