[sethw]

Both the speed and power you get out of a piston is directly related to the bore size. A larger bore will give you more power and more speed. This is because the compressed air is impacting upon a larger surface area within the larger pistons. The extending force of a piston in pounds is equal to the pressure being sent into it (in pounds per square inch) multiplied by the surface area of the disk inside the cylinder (the radius of this is half the bore size). There is a chart listing the force output of the allowed bore sizes in the FIRST pneumatics manual.
The speed at which different size cylinders will fire is more difficult to calculate, as it depends on the flow rate of air into the cylinder. This will depend on the valve you are using, whether you are using flow controls, the inner diameter of your tubing, and the arrangement of your tanks. All other things being equal, though, a larger bore cylinder will fire more quickly than a smaller bore cylinder of the same stroke. The longer the stroke, the slower the extension, typically speaking.
The easiest way to demonstrate all of this to your team would be to set up a small pneumatics system on a piece of plywood or something, and fire two cylinders of differing bore sizes if you have some already. Hope this helps.