What is the best documented result for the average length of the penis? Also, what is the average circumference?
You mean the average human penis, right? - pays to be specific.
Yes a human penis.
Average length of a penis - 5.75" to 6.25"
Average length of a Doper penis - 11.25" to 12.25"
Average circumference of a penis - 4.15"
Average circumference of a Doper penis - 7.25"
YMMV.
Americans have on average only about 3" of penis 
Unless penis ensues.
How did you know?? UP with Dopers!!
On average, I frequently trip over mine when I walk.
What?
Well mine is two inches
From the GROUND!!!
My 500th post ladies and gentlemen. 
Ah, you’ve come to the right place, as I just happen to be sort of an armchair expert on the subject. Not only did I recently start a very similar thread on the subject, in true Doper fashion I went and thoroughly investigated the matter myself and parlayed it into a semester project for an honors statistics course that I was taking at the time. Unfortunately, I can’t post the report here because the statistical symbols and tables won’t appear properly. If anybody wants a copy of the project, just e-mail me and I’ll send it to you. Make the header real obvious, so I don’t discard with the rest of my junk mail. (Or is there anyplace where I can post a 1 gig Word document where it can be made publicly accessible, à la one of those photobucket-like jpeg hosts)?
The bottom line is that there is no definitive answer to this question at this time. Results from different studies have yielded inconsistent results. The closest definitive answer that can be given is that the mean length of a Caucasian penis is somewhere between 5.000 and 6.250 inches, and the standard deviation is most likely between .750 and .825 inches.
Wow, you must be really short 
Does the average include the women
Of course these are limp values.
I think Bippy has already addressed this issue.
I use mine as a car substitute.
Works well as a boat too.
He’s that ten inch pianist we’ve heard about
And you used it for a penis joke! <snif> They grow up so fast!