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.
55g planted goldfish and dojo loach tank
46g & 29g planted tropical community tank
10g planted something tank...still deciding
5g planted Betta tank