The first thing to say is that not ALL black guys have huge cocks and not ALL white guys have small cocks - there is a bit of give and take on both sides of the equation. That said, black men do TEND to have larger cock on average and when they do get large they get very large.
The largest my wife has taken is 11"x6".
The thing here is, the pussy was designed to take a full term baby so stretching isn't the issue so much as impact, anatomy and position.
Not all women are created equal either and some have a much lower cervix than others, some I can reach in and touch, others I've never felt...
For my wife, 11" is very doable but she has to get him "in the groove". By this I mean she has to get his head in the pocket behind her cervix. Once he has opened that up and is driving into it comfortable she's gonna have a great time - and as has already been said he has to take it easy for a minute and find that groove. Once he's there she'll take everything he can give her (make her grunt a bit naturally). Obviously most men don't occupy this area of her pussy so she LOVES to feel a big guy get there. There is also some evidence that there are nerve endings receptive to sperm in that area which can trigger an instant orgasm (to aid conception). I've seen this happen many times; as soon as my wife feels his seed start to arrive in this 'pocket', her hips lift and she cums like her life depended on it.
As for the width? Well she loves to feel stretched and black guys do that for her better than white, and certainly her pussy is in a very different state after a couple of hours with a black guy than with a small dicked white guy.
The first pic below is post sex with a white guy and the second post sex with a recent black guy. Oh yeah, she knows she's been had alright, her pussy is unable to close down properly and is much looser for quite a while.
Oh - and yeah, she screams like nothing I've ever heard when she's on all fours with and 11" monster driving in and out of her without mercy!
)
PS - No that's not the 11" guy - He was maybe her 3 or 4th and he was THICK! Just helps put us white guys in perspective!