I purchased a 2.7 gallon Fluval tank. It is wonderful. However, in my ignorance I did not change the water often enough and my fish very quickly developed fin rot. I now keep in mind the Old Fish Lady's instructions on water changes
Things can go badly very quickly in a small tank. However, a 2.5 gallon tank can be a fine betta home as long as the proper care is provided... that includes the required water changes & water chemistry, a heater, thermometer and filter.
My betta surfs the glass. I believe he would like more room to swim. I will be purchasing a 15 gallon tank and using the 2.7 gallon as a moss ball home and quarantine/hospital tank when needed. 15 gallons is a personal choice for me. Long term it is small enough for me to manage and large enough to use for larger fishes or schools of fish once my betta passes on (in about 5 years
Also, I can always divide the tank if my betta doesn't like the extra space.
I saw 5 gallon tanks, 6, 8 & 10 gallon tanks that would have suited my betta just fine. Bigger tanks = more options and easier water quality maintenance.