If she's 5'3 and 180 lbs then she is somewhat overweight for her height, and if she keeps mentioning it, then it obviously upsets her.
By asking what you think, maybe she is wanting reassurance, but it also sounds like a possible loaded question to me. If you say "yes, I think you could stand to lose some weight", then she can feel justified in acting hurt, but if you say "no, I don't think you're fat" then she can argue with you and say "Yes I am! You're just being nice by saying I'm not" and the cycle continues.....
At the end of the day it doesn't really matter what you or anyone else thinks about her weight. She is the only one who can make the decision either to lose weight or accept herself as she is, and I would be tempted to say something along those lines next time she tries to get you to comment on whether she's fat or not.
Depending on how close a friend she is to you, I guess you could offer to go swimming or do some other exercise with her, if she decides to do something proactive to help herself lose weight? That way you could support her efforts and help her feel that you care.