See The article “Am I normal? A systematic review and construction of nomograms for flaccid and erect penis length and circumference in up to 15 521 men” onlinelibrary.wiley.com
For an erect penis, the mean (average) circumference is 11.66 cm (= 4.59 in) with a standard deviation of 1.10 cm (=0.43 in).
Assuming that the population is distributed “normally” (I.e., a bell-shaped curve), 63% of the population is within +/- 1 standard deviations (i.e., 4.59 in +/- 0.43 inches, or 4.16 to 5.02 inches); 95.4% of the population is within 2 s.d. (3.72 to 5.46 inches); and 99.7% of the population is within 3 s.d. (3.28 to 5.89 inches).
The probability of having a circumference of 6” or more (=3.254 s.d above the mean) is 0.00010, which is larger than 99.999% of the population (assuming, of course,that the population is normally distributed).
Does that answer your question?
(It is amazing what 3 days of corona virus lockdown will drive you to.)