What exactly is your size? I mean, unless you have a micro-penis, you should be alright. Plenty of women out there who are perfectly happy with average.
As for your ex — I think we all run into a woman who at one point or another manifests herself as a total bitch, that’s if you’re lucky - easy to get several after another!! And the way she does that is by targeting something that she knows you are or could be sensitive about, dicksize seems a popular target these days, but it could be anything. The best thing to do in that situation is to simply laugh at the silly cunt for her immature behaviour. :)
Focus on what makes you feel good, not on what makes you feel miserable. Now you can’t control what she says, but you can control what you focus on, i.e. the ball is in your hand.
Good luck!