Totally depends on the fish. It's not about size either but lifestlye of the fish. Big swimmers shouldnt go into such a small tank but there might be a larger fish that just hangs out and doesnt need the swim room.
Check out liveaquaria.com Thye have like every fish you'd ever want and it shows recommended tank sizes and a little bit about care and requirements.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.