Just a few comments from a data analyst
I think that a genuine 7 inch penis is really quite big, and I suspect that the real average/median size is quite a bit smaller for the following reasons.
There is no perfect study of penis size. However, if you take all of the studies done and do a meta-analysis by plotting their statistical power (e.g. how rigorous their methodology was, sample size etc.) against the reported average size, you find a very strong correlation. The results show that the better the study, the smaller the reported sizes; and the worse the study (e.g. some of the self-reported ones) the larger the size. If you extrapolate from this you end up with a real average size of a little less than 5.5 inches.
Also, dont forget that when most people talk about the average size, what they really want to know is the median size. That is to say, what is the size where 50% of men are larger and 50% of men are smaller? All the data Ive seen has a distribution that non-normal and is biased towards the larger sizes. What this means is that the average size is greater than the median size. In other words if 5.5 inches is taken to be the average size, then most men will have a penis smaller than 5.5 inches. Its counterintuitive, I know but thats how the stats work out.
Theres also the quite obvious exaggeration in size as reported by people on the internet. You only have to compare the sizes people report to some of their photographs to see how obvious the exaggeration is. Not to mention the distortion in reported size due to very spurious measuring techniques.
To be fair to these people, they probably feel that they are smaller than the average due to the problems Ive discussed above, and therefore feel that they need to exaggerate in order to keep up.
Conclusions: 7 inches is probably much less common than is thought, and the internet needs more statisticians.