A few considerations for when discussing this matter:
1. Most men, when listing off their penis size, will only announce the highest number they've achieved. Erection rigidity and volume vary naturally with arousal and a handful of other factors, but if my hypothetical 11-inch monster comes in at 10 1/2" when I measure it for the camera, I'll say "Hang on, I'm not fully hard," delete the photo, get myself as rock hard as possible, and try again. We always want to be 'at our best' for the camera/for the ruler.
This is contrary to how research into the average penis size is carried out. The researcher won't be sympathetic if Mr. Average says "Whoa whoa whoa, 5 1/2 inches? I'm supposed to be 6. Give me like ten minutes and a Playboy, maybe a Cialis if you've got any on hand, and then we can remeasure." You're supposed to take the average size of your erection, not the biggest, most impressive measurement you've ever gotten from your hardest pre-drooling boner.
2. The proper way to measure is with the measuring device pressed firmly to the public bone, measuring along the top side of the penis.
3. My statistical analysis assumes penis size follows a Standard Normal distribution like height, IQ, and a great number of other data sets.
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The Ansell, Inc. LifeStyles Condoms study in 2001 gave a higher mean length than a lot of the methodologically-sound studies have, at 5.877 inches with a standard deviation of .825 inches. So we'll use that data set for our calculations to give the biggest statistical chance of an 11-inch dick.
With those numbers, we can conclude that a penis of 8 inches or greater is relatively common, at one in 200 men.
But a penis that is 10 inches or greater is far less common. One would expect 99.999970972% of men to be smaller than that. But what does that mean for us?
According to cia.gov, in 2014, there were 2,656,092,799 men aged 15 or older in the world.
2.9028e-7 * 2656092799 = 771
[2.9028e-7 is the frequency of a 10"+ cock; it comes from 1 - .99999970972]
We would expect 771 men from the demographic of all men in the world aged 15 years or greater, to have at least 10 inches.
I went with 10 inches, because calculations say that we would only expect one person to have a true 11-incher or greater. And it's probably possible for some of the people who would measure at 10 inches or higher in clinical conditions to get it up to 11 inches if they keep measuring until they get the biggest, hardest erection possible. I'd say that the number of people on Earth who can legitimately claim to have measured at 11 inches is about a third of those who would clinically measure in at 10"+, so about 250 men.