Not necessarily. I've had my very promiscuous phase, so I've seen dicks of all colors. I can assure you that black men aren't necessarily bigger, I've hooked up with white guys who had bigger dicks than some black guys, and vice-versa. Same for guys of other races. I don't think there's an actual correlation, or it could be related to other things like height and other non-racial genes.
If I had to remember the biggest dick I've ever held, I'm not sure if it'd be my black FWB or a white guy I saw a few times who ended up being my 1st bareback experience when I was 19.