I agree that the most important thing isn't the size but the way you take care of your betta. A smaller tank is fine if people keep up with frequent water changes and keep the water at the right temp.
Personnally I had one of my betta in a 1.5 gallon, heated. Real pretty tank. He started tail bitting and a couple of weeks later my other betta who was in a 5 gallon died so I transfered him in the 5 gallon and the tail bitting stopped.
I think that quality is important but some fish do like bigger tanks... I guess you have to adjust depending on the fish itself. Bigger or smaller tanks, as long as the fish is happy!
My sister has a betta in a 2 gallon and she is doing great.