I think it's clear from all the threads on this that there are big dicks all over the globe. From my own personal (rather extensive) experience, however, I'd say this is what I've noticed.
1. Well hung men of African descent are relatively numerous. Whether they be American, Brazilian, Haitian or from other parts of the African diaspora. I have noticed however, that their size is sometimes overestimated because black men are often more showers than growers. Which means that their dicks often look very large when flaccid, but may not grow that much when erect.
2. It is also not uncommon to find large cocks on men of European descent. I've seen really big ones on French, British, German, Belgian, Spanish, Italian, Czech, Hungarian, and Russian men. This variety leads me to believe that no one nationality has cornered the market on being well hung. Obviously, there has been lots of mixing of peoples in Europe and among white Americans, so the genes for most characteristics are pretty diverse. Whether the "average" white is hung much differently from the "average" black man is impossible for me to say, but there are clearly plenty of big ones in both groups.
3. I have little experience with Middle Eastern Caucasians, such as Arabs, Iranians, etc. so I can't say. I would expect that they would be similar to Europeans. I have lots of experience with Jews, and I have to say, I've seen lots of well hung Jewish men. So, ethnically speaking, that probably is an indicator for other people of Middle Eastern origins.
4. I have yet to personally see a really well hung South or East Asian. I've seen some moderately well hung photos or porno films of a few. But these can be deceiving. I don't doubt that among these enormous populations there are some big dicks -- normal genetic variation would almost certainly mean that. But I personally have not experienced being with or even seeing one. My experience is also limited by the fact that there are considerable cultural inhibitions in Asia for men to have sex with other men, perform in porno films, or even be photographed nude.
5. Native Americans would be ditto as for Asian. I have little experience with them. In South America where there is a great deal of miscegenation, there are lots of well hung men -- notably in Brazil. The racial mixture there is often so blended between white, black, and native American that it would be impossible to identify many individuals by race or make a guess as to what features represent what racial background.
So those are my observations. Granted, they are limited by my own experiences, so they probably don't mean much. But that's my answer to the original question.