10 Gallon Tank?

[battered]

10 gallons is perfect. Can work as a small community tank as well. The open space might be bad if there's nothing in the tank. As long as the betta feels safe and secure, you're good to go. Bettas can go in large community tanks (considering compatibility issues of course), so 10 gallon is absolutely fine.

[Tikibirds]

I think what may stress some bettas out is if you had a 10G but only 1 small plastic plant in it - ie, big, EMPTY space. If you put in lots of decor such as silk or live plants and a few hiding spaces, I'm sure he will be very happy. it will give him cover and make him feel more secure.