what type of games are you going to be playing and is your gun a gas hog or an efficient one. If you're going to be doing big games and carry lots of paint out onto the field you may want a big bottle, there's no point in carrying lots of paint if you aint got the air supply to shoot it, likewise there's no point in having a big bottle if you're only going to be playing short walk on or punter style games. I use a 1.5 litre 4500psi bottle and when i do get full fills i usually have air left after i've gone through the 11 140 shot pots my vest can carry and a hopper, but i shoot efficient guns though.