There is an exhausting number of "what's average" threads on here but reading through this one brought up a couple thoughts...
1.) You don't need to be stupid to mis-judge penis length. To this effect, I don't think that "women can't judge size", I think both men and women are guilty of fallability. When you are talking about a difference of an inch or two, it is very easy to be wrong. Draw a line of random size on a wall or large piece of paper and guess to within a 1/2 an inch and see how far off you are...My guess is it may be enough of a margin of error to make the difference between "average" and "big", as defined in various threads and posts
2.) flaccid size most likely plays a role too as a partner will most likely not be seeing you with a rock-hard, most length-showing erection the entire time. A cock that starts out at 3 inches soft and grows to 6 inches may, in retrospect, seem smaller than one that starts out at 5 inches soft and grows to 6 inches.
3.) Although I advocating drawing a line and guessing size, a cock is not a line nor is it two-dimensional. Variations in bend (both side to side and up and down) as well as girth, color, etc. effect visual perception. As an example of this, look at Chris Strokes who, to many people, I believe looks much longer than he really is by virtue of the narrowness of his manhood.
4.) Based upon the limited number of surveys conducted (and as reinforced here by a relative consensus) I believe between 5" to 6" in length is a reasonable assertion.
5.) If you think people (generally) bullshit about length, take a look at some of the girth claims out there