Another Betta fish question

I see bettas in very small tanks fairly often. In fact, when I was in college I had a betta named Frankie, and he was in a small aquarium. Is this okay for them or do they need a lot more space? Looking back I feel bad about Frankies small quarters, but it seemed like most everyone had a betta in a small little tank. I know that they have to live alone so maybe that is why their tanks are often small so that people can have another fish live near them and still have a bit of space.