Self-reported vs. clinically measured penis size
When articles state some research which asked men to provide their data regarding measurements of erect penis size anonymously
Men are obviously quite ego driven, it is not uncommon for men to overstate something or exaggerate things. The saying "fisherman's tale" is widely used to describe various claims for a reason. When it comes to penis sizes, it is not surprising that studies which rely on self-measuring reports of penis size constantly give larger estimates of average size than studies which rely on clinically measured penises, up to an inch longer and significantly wider in circumference.
On the other hand, measuring erections in clinical conditions might not convey a realistic setting therefore subjects might not be as aroused as they are in a more natural setting and it is widely known that the size of the erection will depend on the arousal method. Mental stimulation for example has shown to be least effective in getting an erection whilst oral sex has been shown to be among the most effective.
Recent studies of average penis size
Recently the Journal of Sexual Medicine has published a study where findings have show the average penis length amongst adult American men to be 5'6 inches long when erect, which is approximately 14'2 centimeters.
This data has been gathered from an arguably relatively reliable source of men who ordered condoms, where they had to provide exact measurements in order to receive the condoms of correct size.
The average penis girth/width/circumference was found to be 4'8 inches or 12.2 cm.
This of course, doesn't take into account that in terms of condom sizes, penis length is largely irrelevant, the condom rolls over the penis and covers it, it is the width that is important, so it may be reasonable to suspect that the average penis length data gathered in this study might be slightly overstated. There is no reason to suspect that the girth is inaccurate though.
The same study claimed not to have found any difference in penis size in terms of racial background and height. The sample of the study included over 1600 Americans and since United States is quite the melting pot, meaning that it accommodates people of very diverse backgrounds it is safe to suggest that this is close to the average penis sizes across the world.