Fortunately for you, bigger willies on black men is just a myth, at least in the USA and EU anyway. There are no actual studies done on the sizes of willies if the blacks have larger ones in comparison ethnicities and races. Size is relative to anyway, man being man wants theirs to be seemingly big. It is a myth made popular by pop culture and such.
In the USA, African descendant have more than not bred with the Caucasians, so average size is the same for blacks and whites. Since black man is bigger in height in general, the consensus becomes them having bigger willies as well. It could be because they are still more likely to come from the lower income, the myth of their size got spread around to compensate for the fact that they are poor. The stereotype persisted with no real basis. I mean, why would they ever try to debunk it? Then pornography comes along and once again presents them with overly huge sizes as the average. Probably the myth of bigger willies started to spread around by white woman who did the taboo thing of having intercourse with a black man since it is more exotic and bestial in a sense. Then rumours spread around how a black man can satisfy a woman better, with the legacy persisting till today. There you have it, black men don’t have bigger willies, it is just a myth. If it is true, then the size is probably not significantly bigger and rather born out of a racist tale.