Even if they are, you know, fat?

It shouldn't be. It would be like people not being allowed to call me bipolar.

It's really rude to call a chick that..... mainly bc women have historically had value based on how they look.

So, calling some girl/woman fat is like calling her a low worthless POS.

Yes, the politically correct term is American.

Depends on the person. If I know him or her for a long time and I become a good friend then I say it with no problem. If I don't know the person then it's rude.

Of course it's rude. Being true doesn't imply not being rude. And the people who call such people "fat" are often to a large degree responsible for these people being/staying fat.

If he or she asks me, I'm assuming that person can handle the answer. If I'm the bad guy for not lying, I'll just tell whatever PC-crusader that attacks me to go fuck him/her self.

If you go and throw it in people's faces against their will though, that's another story.

You trying to impede upon mah freedums boy?

http://www.youtube.com/watch?v=XwR3e2iq9EI

yes it is

they already know they are fat and might not feel very good about it. some people have a hard time loosing weight

I would never.