Compared to other primates, even larger primates such as the gorilla, the male human genitalia are remarkably large. The human penis is both longer and thicker than that of any other primate both in absolute terms and in relative size compared with the rest of the body.
While results vary across studies, the consensus is that the mean human penis is approximately 12.9–15 cm (5.1–5.9 in) in length with a 95% confidence interval of (10.7 cm, 19.1 cm) (or, equivalently, 4.23 in, 7.53 in). The typical girth or circumference is approximately 12.3 cm (4.85 in) when fully erect. The mean penis size is slightly greater than the median size.
I question this answer. See "The Definitive Penis Size Survey Results" from which I suggest that your answer was derived without attribution: