I know it's hard to experiment because as a newer team you don't have an assortment of cylinders to try. Our first year we were lucky enough to borrow some from other teams, and a 1.5x8" was included in the KOP back then.
We tried (2) .75 cylinders directly connected to a kicker, and got about 10-15 feet. Then we tried a 1.5", which was too slow but had enough force. We also found that you can't 'push' the ball, it had to be hit or kicked.
A few teams have posted successful threads about their kicker designs, you could research their methods.
Or if you get a kicker to work with springs / surgical tubing, you can calculate the force required to 'reload' the system, and order the length and diameter piston required to reload it.
Page 15 of the Pneumatics Manual has a chart of pressure/force for the cylinders.
http://www.usfirst.org/uploadedFiles...%20Rev%20-.pdf
Also some great info here:
http://www.team358.org/files/pneumatic/
Great resource Team 358, thanks!