Wow, I didn't expect so many replies! Thank you all for your input, I find it all very useful. I agree that there isn't really too big of a tank for a betta, although I know it really depends on the individual fish. It's also nice to hear personal stories as examples, so I can consider what the outcome could be. I'm debating putting Link into a twenty gallon long, with lots of plants and caves (fake plants, I'm not confident enough to try live) since he seems to always make use of all the space in his ten gallon.
Kindness and respect should be shown to all animals, no matter if they are furry, scaly, big, or small. Be a hero to the animals and GO VEGAN!