I'd say a five gallon tank would be the minimum size for this. My betta is living in a 1/2 gallon tank, and although it's small he's been perfectly healthy ever since I got him, about a year and a half ago. However, if there's going to be other fish in there you obviously want a bigger tank. Bettas can be aggressive with other fish, so it's important to allow plenty of room. You should get at least six neon tetras to go with your betta, as they are schooling fish and should stay in a school to keep your betta safe. Tetras are fin nippers which is obviously dangerous for your betta, but they're usually okay when in larger groups.