I suspect she wants to have sex with a black guy because she's either never had one, wants to try a different race or she's had one before and wants to again. Likely no different than me. I've had sex with Asian girls, black girls, East Indian girls etc. simply because I was attracted to them, not because they had particular sized body parts.
But you never know, she may believe all black guys have huge cocks which isn't the case. And yes, lots of white guys have big cocks. Then there's what makes for a big cock, length, girth or a combination of both.
Maybe the OP can shed some light on his SO's motivation, we're just guessing.