I'm sure it has been said before, but the entire point of a business is to make money. The porn business is no different. One way they can get money is by spreading misinformation or claims that seems truthful, but really aren't. For example, 11 inch penises and whatnot. Most of the pornstars are between 6-7.5 inches long, with girths around 5-6 inches.
The long, and statistically improbable outliers, are at or around 8.5 inches (that number fluctuating 0.5-0.75 inches above or below that mark). I have yet to see a legitimate measurement of a 11 inch penis, but that is exactly the point I am making. They won't sell anything if they remove the fantasy. Using fisheye lenses or relative spacing to make things appear larger, relatively speaking, is smart business, which is what they do.
I don't know of any natural girths that are above 7 inches, I have heard and seen Shane Diesel, but that penis does not look natural to me. I'd say that that mark is about the upper threshold. Thicker ones may exist, but they are statistical outliers, considering the majority of girths.
In regards to length, the biggest pornstars (ex. Danny D, Napier, Boz, Mandingo, Lex, Rico, Isaiah M, etc.) all fit what I have laid out above: 8.5 inches, fluctuating 0.5-0.75 above/below that mark. This thread (I can't be bothered to read 160 pages) makes that much clear, to a very fine point, I may add. Using relative spacing, they can bump the number up higher. Do you really think it is smart business sense (and in the interests of the actors) to take an accurate measurement by an unbiased third party? When it comes up as 1-1.5 inches longer than the above-average Joe (say, 7 inches long) reading online, he won't feel the need to glamourize the actors and will come to terms with the reality of the situation.