Well, you could have responded like this:
THEM: You're fat! Ha Ha Ha!
YOU: Oh, dear, you are right. Thank you so much for pointing that out. By the way, are you planning on getting your nose fixed? You really should, you know. It's quite unattractive. And that acne-are you seeing a dermatologist for that? How unfortunate for you. And those crossed eyes...oh my.
THEM: Uhhh...derp.
YOU: We were giving each other constructive criticism, weren't we? I mean, why else would you come up to me, a total stranger, and comment about my appearance?
Yeah, I know, retaliatory rudeness and all that. Can't really do that.
I'm sorry that happened to you. It takes a very small minded person to insult a total stranger like that.