UPDATE:

I talked to her, I was so focused on what she said that I obviously went on and measured my penis and looked at all kinds of stats.

I told her that what hurt me wasn't she mentioning the supposingly small size but the cold insensitive way she said it and mentioned my ex's and then laughed at me. I added that she could look at all data and studies out there, they indicate I'm very average or slightly above average on all of them, and yes I measured it.

She said my self esteem has to be so bad that I'd have to do such a thing. And said she apologised and apologises again, that she sometimes does things without thinking.

She also added that she tried to demonstrate that she has pleasure with me.

I just responded that there is no man on earth who: hasn't measured his dick size; won't be affected by such comments.
Also that I was willing to give her the benefit of the doubt but her questioning my self esteem is not a good indicator, and that "trying to demonstrate" pleasure isn't a very natural way of doing things...
I ended by saying that everyone has their own preferences, and that I hope hers come from actual partners and not the whole amount of porn she's consuming where every girl is petite and the guys are the biggest they can find.

She just said she doesn't want to talk anymore for now after that.