Your proper penis pump length should be 2 - 3 inches longer than your erect penis size. So, if your size is 7 inches when erect, you should use a 9 - 10 inches long cylinder.
For the diameter, you should give at least 1/2 inch of allowance around your shaft. First, measure the size of your girth (when erect). Wrap a measuring tape around the thickest part of your shaft and mark where the tape meets to get the circumference of your penis. Chose a cylinder that is around 1/2 inch bigger than the diameter of your penis.