Well if you just want some shrooms and maybe a snail or two you could probably get away with a couple of gallons. That probably wouldn't be too much fun though so I'd probably go for a 5 or even a 10. That way you could get a crap or two or maybe even a small fish like one of the small gobies.
As an alternant you could also get some more critters in there like more softies, crabs and shrimp or even a gobie or other smalish fish. You could have a lot more room to play with your tank if it were a little bigger . Also the smaller the s.w. tank the less stable/more attention you have to pay to it.
hth
chris