Fully erect, without any restraint, I am just under 8-1/2", and thick. My cock-head is larger in diameter than my shaft by about 1/2".
In our decade plus of doing this (hot wife thing), I've seen hundreds of cocks, some bigger, some smaller, some thicker, some thinner.
My wife picks encounter partners based on the appeal of the cock - she demands smooth, completely shaved partners, who are circumcised, with what she calls "pretty" or "beautiful" cocks - deeply veined, big cock-head, and a few other qualities.
She also likes a skilled partner, with stamina, who can control himself. That is a harder to find quality.
If I were to pick an average, most of the cocks my wife selects fall into the five to seven inch range, with the majority probably in the five-six in range. Most partners we find can last twenty minutes before cumming if they are good, some a little longer. Refractory periods vary from five minutes for some younger guys, to a half hour, with increasing stamina after the first cum.