Ideally, you want a cage that is just long enough to allow the tip of your penis to come to rest at the end of the cage.

Take a ruler and place it on the underside of your flaccid penis, measuring from tip to base. Do not press the ruler into your scrotum, as this will give you an incorrect measurement. Just allow the ruler to make contact with the skin there. You should be completely flaccid when taking this measurement, but not the “retracted inside the body because it’s cold or scared” type of flaccid. For accurate results, take this measurement several times throughout the day, to find your most common length. When you have found your measurement, deduct a ¼ to a ½ inch off the final results and this will be your ideal length for a cage.