Wouldn't recommend any smaller than a 5 gallon, even for a betta. They like to stretch their fins, and water quality is easier to maintain, along with temperature. I've had a betta by himself in a 10g before and he loved it. Just got my 5.5g set back up for one whose fins got mangled somehow, poor guy.
100g planted community tank
75g planted goldfish tank
29g planted betta community tank
5g planted Betta tank