16 or 20 oz
if you're going to stay with co2 you have to get the 20 oz and i would get at least 2. Remember in the summer you will get, if you're lucky around 700 - 800 shots, but in the winter this is considerably reduced. Bottles freeze down reducing their pressure and amount of shots, and if your bottle freezes in one game you will not be able to use it in the next game.
When i was running co2 i had 4 bottles to last me the day, there's nothing worse than starting a game and your bottle drops pressure. If you can afford it go to air, you know it makes sense !!.