It's an individual thing. Some guys like fat, some guys like chubby, some guys like curvy, some guys like normal, some like athletic, and some like skinny. You are under no obligation to date someone you don't find attractive, just don't (unless they push the issue) make it about them being fat. A simple "I'm not interested" is okay, no explanation required.
I am fat, and alot of guys are attracted to it (I'm surprised how much I get hit on, especially because I don't like my body, and usually don't dress particularly nice).