Depends on the fish. For dwarfs, and some wrasses, I have a 20 gallon that runs all the time, fish or no fish, with some sand, rock and PVC fittings, hang on the back filter with sponges, small power head and just a small flourescent lamp, enough to keep algae growing on rocks and glass. The rock and sand does absorb some copper, but not much and I have found it seems to calm the fish by having it. I also always have an ammonia alert badge (Seachem) in the tank.
For larger fish I usually go with a 60 gallon, but sometimes 100 gallon.
What ever tank I always make sure that there is an established biological filter. Either in the tank or I take some from other established tanks.