[DonRotolo]

1. It's that simple. 2 cubic inches of air at 60 PSI = 1 cu in of air at 120 PSI.

Step 1 is to determine the throw of the cylinder you need. Round up to the nearest inch - it is easy to limit throw, but it can't easily be increased
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Step 2 is to determine how much force you need. 60 PSI acting against 1 square inch (or piston area) gives 60 lbs of force (approx, there are friction losses). Use the smallest bore you can, since it uses least air. Note a larger bore can deliver more force, or the same force if the pressure is lower.

Step 3 is to Find the cubic inches at your given pressure. a cylinder with a 1 square inch piston that is 5 inches long uses 5 cubic inches.

Step 4 is to guess how many times you will actuate the cylinder. Extend and Retract are each "one time".

Cubic inches * # of actuations = air needed. Multiply by (output pressure / storage pressure) (usually 60 and 120) and that's how much storage you need "theoretically". (Tanks have a cubic inch capacity specified) In reality, pressure drops as you draw against it, so it becomes a differential equation... So just round up and add a tank, empirically it will be close. Then test.