I had a betta in a one gallon vase on my desk. I didn't know better at the time. I wouldn't call it cruel. We kept each other company. It had a peace lily at the top with roots that dangled down. He would often explore the nooks & crannies of the roots, at a very slow pace. And I'd change his world around every week or so to keep things interesting. Even turning the vase just 90 degrees would change the rooting & lighting to him. He lived for 5 years in it which is pretty good for a betta.
BUT, now I know a little better. I've got a 10g planted aquarium with a female betta and 4 albino cories. The difference in behavior is night & day. She is much more active. Sometimes she swims up & down the length of the tank. Other times she's exploring her much bigger world of plants & driftwood. Every time I'm doing any maintenance in the tank, she's within a couple of inches of my hand, always curious about what is going on.
IMO, a bigger space will bring out more of the qualities of the betta. But if you do go with a smaller space, then you would have to work much harder at enrichment.
