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.
Source: http://en.wikipedia.org/wiki/Human_penis_size