In the real world, true 8 inches by 6 inches (hard) is exceptionally rare.
Speaking in terms of erect dicks, guys with 6 or 7 inch cocks often refer to it as 8, and guys with 8 often refer to it as 10 or 11. Complicating matters (and expectations), porn marketing will typically refer to a true 8 inch cock as 10.
In relative terms vis-a-vis people one will encounter in the real world, 5 to 6 inches in length is "average," 7 is "large," and 8 inches by 6 inches is "monster."
And by-the-by, race doesn't really have that big an impact on these numbers -- to date I have seen exactly one fellow of African decent first hand with the mythical monster cock (in his case about 8 inches hard, maybe 5 around at its thickest point), and I've seen quite a few -- the average is the same as caucasians and Asians. And one of the largest I've seen (again first hand) was on a fellow of Asian decent. Across the board, the average, large and monster numbers above apply to all races. People who propegate race-based myths about penis size are at best ignorant, at worst just bad bad people.
True 10-inch-plus cocks, and that's real inches, not "Internet inches," are one in well over a million, and might be most appropriately termed "freaks of nature." To anyone who says he's either found one or got one, more power to you honey, but realistically, the chances are better of winning the Powerball. More likely someone is jamming that ruler REALLY hard into where his crotch joins his let to gain an extra inch or two.
I'm 8 inches long by 6 inches round, and to date I have seen maybe two first hand that are slightly longer, maybe 8.5 or 9. I can't remember ever seeing one that's thicker. Not to say they don't exist.
Motivations for disputing these real world facts (backed up by a lot of straight women, who tend to be a little more jaded about claims of 10 inch this and 11 inch that) vary. Perhaps some guys enjoy telling friends as well as strangers that they've been with fellows who have 11 inch dicks because they feel it increases their desirability -- like hell, if a guy with a dick that big wanted to be with me, then I must be desirable, right? Some fellows with 6 inch dicks make claims of 8, fellows with 8 make claims of 10 -- for reasons which require no explanation. All these bogus claims floating around have had a big impact on what folks perceive to be "average," "large," and "monster."
And then there's the simple fact that the right camera angle can make a 6 inch dick look as though it's larger. It's not very hard to do, and is standard operating procedure in most porn pics and vids, even the amateur stuff. (Hint: Low camera angle, or view from above, both which skew perspective and appear to add a couple of inches.)