It depends, you can't group populations based on skin colour for something like this. Penis size varies even within races. Of "Asians", Indians have the smallest on average. Of Black groups, those from the Congo have the largest penises. Of White groups, those from western Europe have the largest penises.
Overall, Congo has the biggest, with an average penis length of 7.5 inches.
But then you can mix things up by looking at the sexuality of populations. On average, straight males have a penis length of 5.5 inches. But gay males have penises 1 inch longer on average, at 6.5 inches.
You'll find groups of whites that have smaller penises than some asians, and some white populations that have larger cocks than some populations of blacks.