If you can return it for a part-exchange/refund and go for at least 14 litres, I'd suggest that.
2.5 gallons is pretty small and while some people do keep bettas in smaller tanks, I'd always say the bigger the better (within reason). Some of my boys are in fairly small tanks but I have enough tanks that I can move them around and find out which fish like what tank.
2.5 gallons is pretty small and while some people do keep bettas in smaller tanks, I'd always say the bigger the better (within reason). Some of my boys are in fairly small tanks but I have enough tanks that I can move them around and find out which fish like what tank.