-2.5 deviations below is considered a micropenis, so +3 deviations above would be a macropenis I guess.
A lot of it comes down to measurement methodology too. Improper measuring technique can lead to 2" more in some cases. I think it only really makes sense to measure NBP but just kind of slide your fat up if you have it, since there's no point in measuring how much your ruler bends up in to your belly.
As we've seen in the $2 club, "I don't believe 12..13" threads, etc. even on this site, a true, verified 8"+ is really, really rare. There are plenty of guys (myself included) who can get well above 8" if they use more optimistic measuring techniques, but to compare against a study you need to follow the same methodology that they did.
Since people don't like to generally think in terms of methodology and statistics, the result is usually huge arguments back and forth about what exists and doesn't exist. I think the truth is, that you'll probably see plenty of 9-10" dicks using one measuring technique, and almost none using a different one. The guys who show up in the 7-8 range in medical studies are probably at the same size as 9-10" in self-reporting. They have a huge dick in either case, which is why I think looking in terms of deviations is way more valuable than just raw numbers. It gives a better sense of just how big you are, while just knowing the inches would only be more useful if you're trying to measure out space for a cabinet with it.