I felt the need to re-comment on this because I don't think I really answered the question well. Mind, everyone is going to have different opinions on this, but I think that virtually everyone on this board can agree that 5g is a reasonable size to have a betta in.
Some people think you can go lower, many suggest bigger (you should see when I had one of my bettas in 20 gallon... pure joy)... But I think virtually everyone can agree that a filtered, heated 5 gallon tank would keep a fish happily. I think this should be the baseline to start with, it's surprisingly affordable to get a quality 5.5g going vs a tank you'll want to replace half of in a month.
It shouldn't be about only what works for you, but you really need to consider what works for your fish. What makes them happy? I know, once someones heart is set on that oh-so-cute tiny tank, it's hard to change it because it works for them... But when you're evaluating your options, consider your fish as well as your needs. They're a living creature, and will reward you with their amazing personalities & appearances if treated well.