Em, correct me if I'm wrong, but the OP is referring to what overweight ladies should be "called" in the sense of "a descriptive adjective referring to their appearance," not a "replacement for their name."
He's not suggesting we replace people's names with an adjective:
"Hi, Fatty! How's it going?" instead of "Hi, Cassandra! How's it going?"
but rather this scenario:
"Do you know Cassandra?
No, I don't. Which one is she?
She's the one over there, that tall one with the brown hair, who's slightly...em...overweight."
Like it or not, our weight (be it fat or thin) is one of the things people will most remember about us and will use when describing us to other people, along with hair colour and eyes. So I think that what the OP was trying to say was, in this scenario (descriptive adjectives being used to refer to your person)
what word would you prefer to be used when describing you?? It's not a "label" or a replacement for your name...just a means of identifying you in a crowd.
In my case, I'd go with "curvy."

I would find it offensive to have my weight mentioned in that context (or any context), so the need should not arise. There isn't a polite way to do it at all. If you are going to be rude, be rude, but there isn't a polite way to be rude.