The short answer is no. There is not a lot of scientific data on penis size with relation to race, because, lets face it, its not a very "professional" hypothesis. The few studies that have been done (most notably the Kelsey study) have shown no statistical difference between any race. These studies have sometimes found that one group (it differs with each survey) are 1-2 centimeters larger on average, but all of them have suffered from a selection differential that skews results (ex. One study had hundreds of white male responses and 33 black male responses). The average adult male penis length is accepted to be ~5.12 inches. Also, female studies have shown the average vagina to be 4-5 inches deep before reaching the cervix, and surveys of women show that the "optimal" penis length in their opinion is seven inches. So scientifically there is no difference.
Socially, there is the well-known black penis myth. It is interesting to note that most cultures have a "this race is bigger" myth and it is not always black people. South America has an interesting fixation on Asian penises. Also, some radical Islamic clerics have used references to "large" white Western infidels to stir their Middle-eastern following. If a popular belief is only adhered to by a small percentage of the world population, one has to question it.
Also, the only "evidence" consistently cited for this myth is the pornographic industry. First off, the porn industry should never be referenced as a reflection of the average, usual picture of sex. If that were true, all women would love it up the butt and pizza delivery guys would be having sex twenty times a day :) Anyway, you actually see a wide variety of penis sizes among all races, just not many small penises from any race. The few more "extra-large" black actors can be explained as simply market demand. For various social reasons, there is the market for that sort of interracial pornography. And sometimes, it isn't even large black men but fake plastic penises (ex. Monsters of Cock, etc). In fact, the preference for large penises in black men has been decried as a racist tendency of the industry by some black performers. I.E., while white actors are hired on the basis of penis size, acting ability, and overall sexual prowess; black actors are only hired if they are particularly gifted, with no consideration to other relevant talent.
All of that said, people will be attracted to who they like. Different people have different preferences and no one can change that. I know three women who have admitted "going black and coming back". The only concern anyone should have is finding someone who you enjoy being with everywhere, not just in the sack. As long as they can usually please you sexually, there is nothing to worry about.