I always thought it was a no-no to compare exes in general, much less something as sensitive as penis size. She should know that this is BAD behavior. Most women know this can be a sensitive subject with guys and to steer clear.
If my boyfriend started comparing my body in any way to one of his exes, I'd have a HUGE problem with it.
Sexual details about exes should stay in the past. Or at least be on a "don't ask, don't tell" basis.
Hopefully you'll get to the point where you can now start undoing the psychological damage your gf has been doing to you.