There is a book called “Is Size Important” that I am surprised, given the interest in the topic here, doesn’t get mentioned. The author, a PhD researcher, looks into the issue of penis size from a variety of viewpoints.
In one part he looks at reputable published studies. First, he differentiated self measured, researcher measured and third party measured studies (these generally on behalf of a researcher that is not present). Consistent with what you might expect, the self measured are inevitably higher. I think there is a consensus on this board, not mentioned in the book, that self measured tends toward BP and those measurements taken by another tend toward NBP.
Integrating the available data, he reaches a number of conclusions based on the stats. First, he estimates the average erection is 5 1/2” long and 1 3/8” in diameter. (Unfortunately, most of the info is in terms of diameter, not girth). He goes on to say that a man over 6 1/4” can feel certain he is large and a man under 4 3/4” can assume he is small.
As far as the “does it matter” question, he draws it all together pretty well in the end. Most men would like to have a larger penis. Few women like cocks at the size extremes, though most express a preference for penises in the 6” - 8” range - above average. Virtually all female fantasies of cock - either in erotic literature or in fantasies expressed in interviews - are of large pensises. Sounds like it matters to me.
This is a pretty concise book and he addresses some other topics, like race, correlation to other body parts, physical stature, etc. I think he got it as close to right as anyone so far.