I just want to add that 2.5g's can, indeed, be filtered and cycled. I have a filtered 2.5 gallon and a filtered 3 gallon. They're heated, the pH and temperature are stable, and the ammonia/nitrite are always 0 ppm. Smaller tanks can be very stable, suitable homes as well. There's a lot of very small heaters and filters that will fit just fine on a 2, 2.5, 3, or 4 gallon tank.
Also, +1 on big tanks not being best for all bettas. I have a partially blind, very heavy finned over halfmoon in my 2.5g. He needs to rest for a while after swimming just an inch. Even in a sparsely decorated 2g, he had a hard time. For him, a gently filtered, heavily planted small tank is the best option.
On the other hand, I have a betta in a 20 gallon long. It took him several weeks, but he's acclimated to the space very well. He has a high-flow filter and very active tankmates, but he doesn't care much. He swims around the whole tank (although he does like one corner best), and certainly likes the larger space. However, he did just as well in a cycled 2g and a divided 5.5g.
So, my point is, one size doesn't fit all. Just because a larger tank is potentially more stable and the betta swims around the whole tank, it doesn't mean that it's the minimum tank size for all bettas. When you come down to it, they're extremely hardy little things who don't need all that much. A stable, warm, preferably cycled tank that's a reasonable size will suit most of them just fine.
