First of all, we have to clarify that you're talking about a generalization, and that this in no way says anything about any individual white or black man.
Secondly, I have only heard stereotypical/anecdotal evidence about "black" mens' penises. Please send me a scientific article about penis size and skin color, if you can find one. My guess is that no such article exists.
Thirdly, we have to mention that many men have mixed heritage...
That being said, there are a lot of theories that point toward the origin of ALL humanity stemming from Africa, thus we are all Africans...
Fourthly, we must indicate that skin color stems from the amount of melanin in the skin, which is protective of humans from the sun, therefore race comes down to the fact that people of African descent are more sun-tolerant than people of European descent.
That being said, we have said nothing about people of Asian descent, although they are in the majority in the world, if you compare them to European or African people.
As for physical superiority, in the United States, historically, African men were slaves, which were bred for their physical characteristics. This is a heinous practice, but nonetheless, it is an historical fact. This would account for SOME African-American men being larger/stronger than SOME men of European descent.
Finally, it must be mentioned that the author's personal preference is a man with skill, which is not measured by size.