From my experiences, I think that the "size" of the erection is not simply length, alone. Nobody I've seen here has yet mentioned this expression from which you are able to measure size. So first, I'll explain, and then I'll give you my opinion as to race and size
I think there's a different way to measure that's better. Try this for a change. Measure your average circumference and square that figure, or in other words, multiply it by itself, then multiply that by the length, and finally multiply that product by .08, and you get a figure in cubic inches that represents the displacement of your cock in a woman. D = C (squared) L x .08. This is what she actually reacts to, at least, initially. (She can be re-educated to like just about anything, actually.)
Women love to be filled up, but if the penis is too long, or if it is too fat, it is less than erotic to her. She doesn't look at it quite the same way that we do. It's better to be a little less than ideal than too big, in that case. But the thicker you are, the less length she is able to take, so you bottom out sooner. If you have a long skinny erection, you stab her cervix every time you thrust, and some women don't like that either.
What I am going for is a perfect balance (to my mind) between length and girth. I am a little too thick right now for my length. This simple equation shows that a very small increase in thickness increases the displacement in a woman more than length alone. The length increases the mass of the penis linearly, while the girth increases the mass of the penis by the square of the change of girth.
So back now to the question: I think that average black penises tend to be longer but skinnier, while average white penises tend to be shorter and thicker. But were you to take a measurement, like how much water such a penis displaces, the top .025% of white penises would displace more water than the top .025% of black penises.
Then again, we have to realize that there are about 3 times more whites than blacks to choose from, so that isn't a fair sample group, is it?
We are very impressed with length, which is by far the most impressive-looking and photogenic. However, when women have a choice, they will chose the guy who has about an inch more than they can take, length-wise, and who can fill them up, side-to-side. That way, that penis can drive them mad in any position. They want that friction and a strong, solid cock, I think.
There is no question that the black race seems to represent the extremes in the races, physically. They seem to represent the longest, tallest, fastest, quickest, most athletic, etc. etc. And that's fine. But there are also extremes in every race, and since the black race doesn't have the largest number of males by a long shot, the overwhelming number of males in the white race will always win the size contest simply because there's so many more of them.
I am a white male and don't think we begrudge any race their strength. I don't. I like to see it, frankly. It's good that they excel. On the other hand, adult entertainment stars with big cocks are almost always the blacks and I think they pride themselves in that and put a higher value on their mystique, whereas white men, some of whom have as large or larger hard-ons, have a different nature and are not so inclined to even get into adult entertainment. White guys seem to have nothing to prove, and black guys seem to enjoy what they see as their superiority.
There are white guys with such huge cocks that not even the biggest women want to try to take them! Those same men eventually lose interst in sex, being turned down all the time. The producers also know that once they showcase these guys then their hunt is on to find some woman who wants to screw them. That's a problem, because now you've got a guy that nobody can watch anymore because no woman can manage him. So he isn't hired to begin with. They aren't going to whet everybody's appetite and then not deliver.
So we're back to square one. The best cock (to a woman) is the best proportioned, hard and long-lived cock that fully fills her up, brings her to orgasm every time, and gets her begging for more. The most photogenic cock is the long, hard, thin (proportionally) black one. But that isn't the one women usually prefer on a daily basis, except to make money on, in a porno flick.
I think the blacks, overall, beat the white guys in length, but mainly because they are driven to compete that way. I say, more power to 'em. Frankly, I like to see a powerful cock, far more than a huge, floppy one.