Actually, there is some correlation - which is why the “myths” have perpetuated. But the correlation is not so much with physical size, or hand & foot size, nose, adam’s apple, etc.
I’ve heard and/or read a number of “authorities” (urologists, sex therapists, prostitutes, promiscuous gay men, nurses, etc.) confirm racial correlations - but it’s not to the extreme that many would imagine.
The largest number of all men, regardless of race, are around 6” erect. The overwhelming majority of all men (at least 80%) are between 5-7” erect. Blacks usually sport a larger flaccid (or so my informal locker room observations, as well as those of other guys - and a large number of nurses I know, would seem to confirm). The differences tend to be at the extreme ends of the scale. A disproportionately large percentage of “huge” penises tend to be black; and few blacks seem to have tiny penises. Whereas the smallest end of the scale (<5”) tend to be White/Asian. It’s also commonly known that the condoms sold in the Asian market tend to be smaller than those sold in the West.
That’s about it. The 2 most hung guys I’ve ever known were white, but most of the ridiculous flaccid hangs in all the locker rooms I’ve ever been in were black. And just about every “monster cock story” that I’ve ever heard from my ex’s numerous nurse friend/coworkers was about a black guy. Furthermore - and I know this isn’t “scientific” - but I’ve never heard a story about a black guy with a tiny cock; average, yes, but not tiny.
So, behind every stereotype there tends to be varying degrees of truth, else they wouldn’t be spread. However, stereotypes error when they attempt to lump all members of a group together, and stereotypes also tend to exaggerate differences as if in some feeble attempt to justify their existence.