Remember that bigger cylinders come at more expense.
Use the shot calculator to estimate consumption - the shot count won't be 100% right for a particular gun, but it will give good relative comparisons between sizes and pressures.
Enter the cylinder capacity in cubic inches
http://www.scubatoys.com/paintball/paintballshots.asp
EG
standard 3000 psi 0.8 litre = 48ci
1.1 litre = 68ci
1.5 litre = 91ci
Due to the capacity you have just under 50% more when comparing 0.8 and 1.1, or nearly double when comparing 0.8 and 1.5
(Actual cc capacities may vary - remember the litre capacity is a rough value, if you want to know for sure then check the cylinders markings - but you may find a conflict such as 88ci and 1.5 litre marked on the cylinder, which don't add up so it would realy be 88ci and just under 1.5 litre)
At £30 to £40 for a standard cylinder and £150 for a 1.1 or 1.5 then you could get 5 times as much capacity for the £150 by just buying multiple cylinders.
Think about the games you play and the paint you carry.
At normal games such as rentals or walkons you tend to play for 15 or 30 minutes, sometimes an hour
You will be eliminated at some point so may not play the full game
Some sites these days have fill stations in the field, either as mini safe zones or by bringing out a scuba cylinder and fill rig.
If you can go out carrying the paint you will shoot, and the air to shoot it then you are ready
When you go back to the safe zone air up and pod up before you socialise and tell your war stories - or pod while talking