The Kinsey Institute's Penis Size FAQ explains:
There is a much wider range of size in men's penises when flaccid, with the average ranging from 2.5cm to 10cm (1 to 4 inches). In general, smaller flaccid penises lengthen at erection by a greater percentage than do larger flaccid penises, with most men reaching an average size of 12.5cm to 17.5cm (5 to 7 inches), so the flaccid size of a penis is not a good predictor of erect size.
They do not link to a particular study, but provide general references, including to
- Jamison, P.L., and Gebhard, P.G. (1988). Penis size increase between flaccid and erect states: An analysis of the Kinsey data. Journal of Sex Research, 24, 177-183.