I keep my betta in a 31,5g tank (120l). He's healthy and happy, he's eating well, making nests and exploring the whole tank. I don't think a tank can bee too big, if only it's decorated right. I use a lot of plants and floating plants and moss. I also have coconut caves and big roots.
In Norway experienced people recomend atleast 15,5g (60l) for female bettas, and community tanks. I keep one male CT in a 5g tank, and would love to give him more room. I think that it's important to not have any other fish with the betta in such a small tank, exept maybe snails and shrimps. But, I don't think 5g is too small for a betta. Bettas has different personalities, and I think some of them feel safer in a smaller tank. I would not use anything smaler than a 4g for my bettas. For me the ideal size would be 15g.
I have also kept several male bettas in a 53g. They were not stressed out at all, and I have never had problems with any disease or tail biting.