Libresse is a manufacturer of various female hygiene-products. One their Norwegian homepage they have a "Questions and Answers" section where young girls can get answers to questions they have about their body and development and puberty and all that. I will now quote one of those questions, along with the answer. Note that the question seems to be a translation from a question submitted and answered in some other language than Norwegian (Libresse seems to have homepages in many countries).
Question: «I know that this probably sounds very stupid, but it worries me a lot: Exactly how WIDE is a boy's penis? I am very worried, as I have noticed that my vagina is quite small.»
Answer: «A flacid penis can measure from 1.2 to 4.7 inches, it depends on how warm or cold it is. An erect penis can measure from 3.5 to 9 inches; the average is 4.7 to 7 inches. Radius is normally 4 to 4.7 inches. Your vagina will be wide enough.»
Translation or not, it seems that someone needs to learn the difference between circumference and radius....
amused