yeah its fine. like most things the bigger the better but there are commercial units that are only 3 gallons and some of the bigger ones top off at about 5gallon. it really boils down to what you want to keep in there, really. if you going to do a mini DSB
it might be a little cramped in there, but if you wanted to do live rock/algae that would be fine. the design (not volume) is really the most important thing, you should try to make sure it gets a flow of fresh water but not so all the water in the fuge is turned over all at once, you want the water to stay in there for a little while, have some bleed off and then replenished with fresh water from the tank.