Okay, sorry Anna, but people are going kinda crazy here. It is the general consensus that 1 gallon is the BARE minimum for bettas. But some people don't tell that to new fishkeepers and tell them to get a big ol 5-10 gal MINIMUM. Now, it's fine and dandy to keep your betta in a large tank, in fact, it will enjoy it, but that is by no means a minimum tank size. 1 gallon tanks must be cleaned twice weekly and are difficult to heat, so you normally say to just go out and get a 2gal, those need to be cleaned once weekly. I have MANY 2 gals and my bettas are doing fine. The only thing is you have to heat them, because bettas are tropical. Without heat, they are much more susceptible to disease and become lethargic. You can buy a 25watt heater and that will keep your 2gal heated fine. Just be sure to clean out the water 100% once weekly, and keeping a betta in a 2.5 gal tank is perfectly okay, don't worry. lol
Also, bettas in the wild lived in shallow (but by no means small or dirty) rice patties/streams. That is all. The water was clean. Plus, those are WILD bettas, not the domesticated betta splendens you see in pet stores. The bettas you get in a store have never lived in a rice patty, nor will they.