I would think that as long as the tank can be heated, you don't have to go super large. The 3-gallon equipse set-up comes with a filter and all it needs is a heater. For that size, you can add a 7.5 watt heater and keep the betta warm enough because the tank comes with the light. With a tank around 5-gallons, if you use a higher wattage heater, such as a 25-watt, you do not need a light. I have a variety of set-ups and all my fish seem happy (actually each of my five betta have entirely different set-ups, aside from two having the same filter).
Oh, and water changes are very stressful to bettas. I've learned a trick to keep them happy. After a water change, they get their blood worms. A couple of my bettas use to pale up after the water changes, but with anticipation of the treat, they no longer do so.
ETA - keep in mind (at least in my experience) that bettas with larger fins need more room to move around. One of my double tailed bettas had a difficult time in the 3-gallon and I had to move him up. In his place, I put a female who moves around like a little torpedo. She handles the size of the 3-gallon much better.