Bettas actually can survive in bowls. I currently have my betta in a .5 gallon . Anymore would probably stress him out.
Plus, for heavier finned bettas, smaller is sometimes better.
I feel like anything of about a gallon is good as long as it's being taken care of. Although there is no possibility of it being cycled, getting cleaned every other day works.
I had a 1 gallon with my first fish in it. He lived there for about 7 months before I moved him and put him else where, so techincally, anything is okay for a betta, but the bigger the better.