In the DC Metro area, which we'll define for this little exercise as the District of Columbia, the City of Alexandria, Arlington and Fairfax counties in Virginia, and Motgomery and Prince George's counties in Maryland, there are estimated to be 502,626 men aged 18-44 (U.S. Census Bureau, estimate as of July 1, 2002).
Now, a recent survey (Wessels, H, TF Lue, and JW McAninch, 1996, Penile length in the flaccid and erect states: Guidelines for penile augmentation, Journal of Urology, 156: 995-997) reports an average erect penis length of 5.07 inches with a standard deviation of 1.14 inches.
Assuming a Gaussian distribution, the confidence interval is erf(std.dev./sqrt(2)). We're interested in the chances of being outside that interval on one side of the bell curve, so we subtract that number from 1.0 and divide by 2.
The result is that 0.000854% of the population has a penis which, when erect, is 10 inches or longer. For our population in the Metro area, this means that there are exactly 4 men with "10 fat inches." And they ALL post to craigslist. While holding said 10 fat inches. What a coincidence.