I'll try to answer a bit more directly, although Kill is right, it's probably been answered a blue-billion times somewhere within this forum.
A lot depends on how you play, but the way I figure, you should get a large enough tank to shoot all the paintballs you would normally carry into a game. e.g. I carry 200 in the hooper and 4x160 round tubes... 200+(4x160)=840 rnd. On my A5 with 20oz CO2, expansion chamber, remote, and an egrip in full auto (12bps), I go through about 1300-1500 paintballs per fill on a warm day. ymmv
Sux to run out of air and still have paintballs to shoot!
I started with a 20oz and bottle on the grip. It can be heavy on a full tank. Plus I shot a lot of liquid when playing that way because I would end up tilting the marker forward and allowing liquid to run up into the marker. Now I run the 20oz by remote and on a warm day can shoot full auto without freezing the marker.
Hope that helps. I'm sure there are some really good pros/cons to be found searching thru the forum.
My 2¢
MG