This blog entry offers a summary of penis size studies. Note that these are not flaccid or stretched studies, but fully erect lengths.
“In 1942, prompted by the “clinical need” “for more thorough knowledge of normal variation in the size of male genitalia,” William A. Schonfeld and Gilbert W. Beebe, performed the first widely-known penis size survey. In 54 men between the ages of 20 and 25, they measured the mean erect penis length as 5.13″. . .
The classic penis size study is “Sexual Behaviour in the Human Male” due to Alfred C. Kinsey, et al (1948), which had a sample size of 3,500 subjects. In this study, each participant was given a stamped postcard against which he would hold his erect penis, mark its length, and drop the postcard in the mail. …In this study, Kinsey found the average erect size to be 6.2″.
William H. Masters and Virginia E. Johnson’s 1966 study was the first to address the “phallic fallacy”, the mistaken belief that size increase during erection is proportional to flaccid length. In fact, they found that the variation in flaccid size is much larger than variation in erect size and small flaccid penises increase more in size that long flaccid penises. …average erect size estimate, 5.2″…
In 1988, Paul L. Jamison and Paul H. Gebhard averaged the data from the Kinsey and Masters studies. … They found the average erect size to be 6.21″, with a standard deviation of .77″, which is to say that 95% of men will not be more or less than 1.54″ inches from the average value.”