What should be the average size of an adult's penis?

12.9–15.0 cm . However, the mean of an erect human penis is approximately 12.9–15.0 cm (5.1–5.9 in) in length. Flaccid penis length is a poor estimate of erect length. Most of human penis growth happens between infancy and the age of five, and between about one year after the onset of puberty and, at latest, approximately 17 years of age.