Well, there's the whole freedom of speech argument, but let's say, for sake of argument, this fat person had copyright over every possible description and depiction of them as a fat person. Even were someone to have such totalitarian hold over their person, I'd still be alright to criticize due to fair use. Now obviously this person isn't a copyrighted work. People's feelings are far less protected than copyright would allow. You can say you don't like someone, say they're dumb, say they could die today and you couldn't care less. As long as what you say is your perception of truth within reason, you can say it. There is no argument in which I actively have no right to criticize a fat person for being the piece of shit they offer to the eyes of the world. There is no circumstance in which me truthfully stating somebody's fat and that's a sign of a lack of self-discipline is not my right. Saying you're a disgusting piece of filth would only even be outside of my legal rights if "right to freedom of body size" or "right to freedom of food intake" were added to the human rights code. No country has yet to amend that to their laws; as such we can assume it will not happen in America for quite a while.