How many gallons is good enough for a betta and is a half gallon betta bowl good enough if not should I get a bigger bowl?

Betas are intresting fish. While for any fish they REQUIRE over 20 gallons, betas dont really care. I used to have 1 beta in a 29 gallon tank. He was in their for a year then I moved him into a bowl for a year. All the stress levels where the same. I dont really think bettas care about space because they hardly swim around and because they are labrinth fish the breath from the surface to get oxygen.