I'm no help...because I would be rude back.
Or I'd just look at her and say, "That's rude" and walk off.
On a side note, I had a neighbor that did this to me--her constant was stopping in the car if I was outside and yelling, "look at that big ole butt," and she really didn't take the hint that she was the only one that thought it was funny. Finally one day she caught me on a bad day and I snapped back, "Better to have a big butt than be the nosey neighborhood B**** that everyone hates." (Which was sadly true.) Years later, jokes on her--I dropped 100 lbs and SHE is the one that porked up. I ran into her not long ago and she was complaining about not having any clothes she could fit in anymore. I politely told her I had some old clothes that were just way too big for me now and she was welcome to have them if she wanted. :P