You should always get the pump that fits you the best, otherwise you will not acheive the best performance. In order to choose the right size unit, you will need to measure your penis, when erect, for both its length and girth. Measure your erect penis with a ruler pressed lightly onto your pubic bone. You'll find a ruler is easier than a tape measure. Measure the circumference of your erect penis around the middle of your shaft. You’ll find a tailors tape measure is easiest, but a piece of string wrapped round and then measured against a ruler after will do. Record the measurement from the pubic bone right up to the tip of your penis.