If she was open enough to mention another guy's cock size in front of you, then that is your license to say "You ain't seen nothin' yet!"
Some women seem to have a double standard and believe it is okay for them to mention their past lovers, but do not want their husbands/boyfriends to do the same. Sometimes my wife, when she is talking to other women, including her daughters, when I am within earshot will refer to "all the men I've been with", which is her way of telling them "I've fucked with a lot of guys! And, I can say so in front of my husband!"
I do believe that if this girl told that her previous lover had a big cock, it is time for you to tell her about YOUR size, even if another woman is present.