On average, penile
length is about 10-15 cm at erection
with a wide variation among men.
After reaching adulthood, penile size remains stable with no further growth no matter what. May I ask what incites you to ask this question. If you like to explore some common issues in male sexuality, go to peruse articles listed in http://www.formefirst.com/onSexLife-ED-CareOptions.html. Best wish to health.