That's unlikely in the extreme. Eleven inches is the penile equivalent of being seven feet tall -- not impossible, certainly, but the percentages are incredibly small. the How did you find this out? Did she tell you? That would lead to a number of possibilities:
1. She's intentionally exaggerating his size for some reason. I don't know why she would do this -- I don't know her, after all -- but she might, for example, be trying to increase her leverage in the relationship by making you feel insecure.
2. He told her he was eleven inches, she accepted this at face value, and repeated it to you. This relies on the ex having told the truth about his endowment -- which, as I noted above, is very unlikely to be his actual measurment.
3. She knew he was big, and, having heard various stories about size, she "guesstimated" him at eleven inches. She might not be aware how spectacularly improbable it is that he was actually that big, or maybe she's sub-consciously trying to take you down a peg, an unintentional version of #1.
4. She measured him at eleven inches. This would seem to be open and shut, but remember that her measuring technique might not be the same as yours. Did she start from under the balls, as I've heard some do? (And which was the method used to arrive at John Holmes' claim of 13", I think.) If that's the case, you can safely take off at least two or three inches.
Really, the only way to be certain that she's telling the truth is to have visual proof that the ex is that big, which won't happen. My own feeling is that she's probably doing number three: he's big, but she's just making an outrageous guess that he's 11". The best thing for you to do is just ignore her claim and show you're not bothered by it if it comes up.