As said before, I think it matters mainly about the preference of the betta, and the fact that the owner takes good care of the fish, regardless of size.
It's also about what would count as a "permanent home". If someone knows enough about bettas and keeps them in a 1 gallon, knowing to change the water every day/ every other day, etc, I'd say that's fine. But to most people, I would tell them that a betta NEEDS a heater, cover, and probably at 2.5 gallon tank or bigger, because I'd rather someone be confident a betta will be more happy than this, rather then them think that keeping a betta in a tiny bowl with no heater, and doing water changes once a week is enough, and keep them in that their entire lives.
I've comfortably kept my fish in a 1.5 gallon, and clean the water often and see no problems with this, though since I will have the money/space, I think I might upgrade him to a 3 or a 5, not because I think he NEEDS it, haha.
This is my fish, Ziggy Stardust, in his 1.5 gallon.