If a heterosexual woman or homosexual man (or bi/pansexual of any gender) has been with more than a few partners, odds are decent they've seen a variety of penises. Most were likely near the average size, but what if an ex-boyfriend or ex-husband or even a one-time hookup had a bigger one than yours? And what if your partner told you it was bigger?
So what? It just doesn't matter. At all. Why are you getting worked up about the size of some other guy's penis?
The biggest thing to remember is that we individually are much more concerned with the size of our penises than our partners generally are. That ex is an ex for a reason. Heck, his size may have even contributed to him becoming an ex. She's having sex with you, now, not him.
That's not to say that she may not think about it from time to time. We all think back on the memorable moments from our past, both positive and negative. It's human nature. And let's be realistic: large penises are very memorable, but not always in a positive light. I'm sure you think back about past partners, too.
Relationships end for many reasons. Incompatible personalities, dishonesty, cheating, bad sex, etc. And very rarely is there a situation where the sex is so bomb that they're willing to stick it out for too long. Every relationship is a compromise (perfect match couples are so exceptionally rare as to not be worth discussing here), where we must accept some aspects of our partner that aren't exactly what we want, and they must do the same for us. For the vast majority of women, penis size simply isn't an important component of their relationship. Hell, most women don't even value dick size that much in bed. It's almost always the owner of the penis that's more concerned with its size.
So I repeat: Her ex having a bigger cock than yours doesn't matter in the slightest. If it did, he wouldn't be an ex.