Research suggests that many men desire a bigger penis regardless of whether they think that their penis size is average or not. This article will look at the science behind the average penis size, how to measure your penis, and the conditions that can cause a penis to be or seem smaller than usual. The average length of a penis is roughly 3. But how did this become the number? There have been several studies over the years that have attempted to give a definite number for the average penis size. One smaller study in the Journal of Sexual Medicine looked at the average penis size in 1, men.