1st Cylinder Size
In order to use your penis pump comfortably and effectively, you will need to select a penis cylinder, a.k.a. the 1st expansion cylinder, 1/4 (.25) inches larger than your erect penis' diameter. For instance, if your penis measures 1 3/4 (1.75) inches in diameter, then select a cylinder that is 2 inches in diameter, that is, 1/4 (.25) inches larger than your erect penis.

To measure for erect diameter, take a ruler and run it across the width of your erect penis shaft. Since the shaft of your penis is round, the exact points of measurement are not going to be touching the ruler. You will need to estimate your thickness (diameter) by "eye-balling" the measurement with the ruler in the foreground and penis in the background.