5 gallons. That can fit pretty much anywhere and you don't have to buy a brand new tank. Check local classifeds in papers, Ebay ect. I bought my 5 gal betta tank for £5. All I needed to buy was a heater and decor ( the filter is built in ) . Silk plants ( not plastic , they are unsuitable for a bettas delicate fins ) are cheap if you can't afford or don't fancy trying real ones, and only need washing if they get a bit dingy looking.
Seriously, you can set up a basic 5 gal for a fairly low amount of money if you source your materials right.
Cheap/second hand 5 gallon
Silk plants ( as many as you can afford ,enough to have several clumps for the fish to hide in and rest on )
Gravel
Sponge filter
25watt heater.
Looking over my shoulder at the 3 gallon moss tank on the desk...I don't know how anyone can bear to put a betta in anything that small let alone smaller.