According to a 2013 research study by Indiana University, and published in the Journal of Sexual Medicine called, Erect Penile Length and Circumference Dimensions of 1,661 Sexually Active Men in United States.
The average erect penis length is 5.5 inches. The average erect penis circumference is 4.8
inches.
An interesting fact from that study showed that erections are larger right after oral sex because this is more stimulating to men, thus a bigger erection as opposed to masturbation alone or with a partner or an erection from a fantasy.