What’s the best response to “you’re fat”?

I saw a post on “what’s the best response to you’re ugly” so I decided to ask this. Couldn’t find anyone asking…

The other day, I was out and about with my god mother and so she invited a friend of hers to dinner with us. That weird man came at me HARD with “you’re fat”. It was the first time we met and the last (obviously). It threw me off hard! I’m of average build, I used to be athletic until I got into adulthood and stopped having much energy or time for sports/exercises. My BMI is on the safe zone and I’m not overweight or anything. I wear size S sometimes even XS for my clothes occasionally M.

I didn’t throw a fit or anything but I’d love to know how would you guys react to this kinda remarks regardless of your own weight. I hate people like these! They think we’re not conscious about our own weight or appearance?? For context, I’m a guy in my late twenties.