the first two are bogus. #3 is the best if you change the grammar. "You wouldn't look good if you were fat."

saying fat is not a proper way of saying this...it is offensive/rude. Replace fat with overweight. the first sentence is ok but not natural the second sentence is perfect (with the revision I offered above) The last should read "You wouldn't look good if you were overweight" Honestly, I don't know if any of these sentences are welcomed/polite/appropriate. Use them with care, said to the wrong person could get you slapped HAHAHA!!

This is too funny though and Melisa is so right. You are sooooo going to get slapped by an American woman. Hahahaha I am dying over here. Hahaha

Perhaps this could be made less offensive by saying "I think you have the perfect figure now. I wouldn't change a thing." This way you are telling the person in an indirect way to not put any weight on by diguising it with a compliment. Hahahahaa!!!! Sorry, I really do not know how to make this "nice". LOL

These are all grammatically correct but extremely rude. As Melisa suggests, I would use the word overweight instead because it is slighlty less offensive. Hahahaha