What I have shared to my female friends is this.
Is that it?
Then to mute/block the person. Though a better response is none at all, and document it and see if there is a way to share it with others so more can band together to stop it.
Now for your mental sake, realize that this phrase represents far more than the size. It represents the whole of the masculine person.
You do know some guys are into fetishes. (SPH, small penis humiliation) around being too small, so by acknowledging it in that specific way you actually increase his pleasure at the cost of your anger.
Lose-Lose for you.
This falls into a comment about an Underlying problem which James Knight wisely highlighted.
As for penis positivity, IMO it comes down to depression and loneliness. When one has to look at anything positive in life that makes you feel good, what do you as a member of the feminine gender pool do? Well you are seeing what men do.
Why are men fixated on it? IMO, down at the core we love making a woman happy and seeing her lose it all, but the big question comes down to the thirst. You and many other women know this thirst very well, but a nice side effect (but not all the same for all women mind you) is that there has to be some connection, ramp-up and feel good. It is an important distinction of having a little firewall between the "on-on" switch for let's do this, and um "should I really".
For a lot of guys there is no firewall, it is like touching a live hot wire.
How do I know this, well as I have shared in the past, I practiced last year and I am doing it again some of the tantric practices of "retention", and I am starting to feel the difference in pleasure. I sense from what has been shared with me with my awesome friends, it is something that when safety and openness are there common with the female response.
Why this share? If guys learned more about their penis and used it in a way that helped them grow, then they would be less focused on the dimensions and more on the feel and touch, which would benefit women "a lot".
As some people can detect, I live in a challenging relationship with my wife as the compatibility isn't there, so it requires me to research and think and feel and work on it a helluva a lot more. I do know whomever my next partner is, I believe they will be in for a fun surprise since there will be a very holistic approach to it. Your favorite friend and author Ed and I are very similar about this, which is fun to see.
Shame never works well and in fact IMO will cause a massive blowback, but I do agree if you do know the person, sharing it with one of their close friends should do the trick, god help the person who has an open timeline on a social media, because wouldn't it be an awesome "share".
But for the unknown wolf's that come and take a bite from your sanity and soul, if you have their details, save it in a folder and maybe there is a sharing site women and men can put up so the details of the perp can be seen.