It's really the personality of the fish and it's caretaker that decides the size of the tank.
Sometimes, it's really just the fish. lol
I upgraded all of my males from 1.5 gals to 2.5 gals when I set up the cabinet, and one of the boys, who has always had an issue with anything bigger than a 1.5, decided to eat his tail. UGH! I just wanted a nice tank with some room for him to move... Brat.
I tricked him though. He's back into a 2.5, but it's a jar, so it has a small footprint, more height, and painted the sides so he can't overwhelm himself. I also agree that 1 gal is a minimum, I prefer the fish to have some swimming room once I've finished decorating their tanks.
I've never been ashamed to admit what size tanks my fish are in, and if anyone ever wanted to try guilt-tripping me on size, they're welcome to foot the bill so that their opinion would actually count. My house, My fish, My rules. As long as my fish are healthy, that's what matters to me.