I said I thought a gourami would probably be happy in a 10 gal by itself, and even better off in a 20 gal. They are pretty active fish. If you plan to keep him with other fish, I would say the minimum is a 20 gallon fish tank.
FYI: Until recently, my male betta was in a 20 gal tank and was the most active fish in there. Please don't keep your bettas in bowls, they are so beautiful to see actually swimming around a tank.