The stereotypes are generally based on fact from my research.... that being said, there are many white guys, particularly of northern European ancestry with really long and big cocks -- 9 inches, thousands with 8 inches, and millions with 7.
White porn stars Jeff Stryker and John Holmes among others have / had unusually big dicks.
The average sized erect human penis (average of all races) is between 5 and 5.25 inches.