How do you say you are fat in French?

Tu es gros.

Tu es grosse.

Vous êtes gros.

Vous êtes grosse.

Use "tu es" (sounds like: tew ay) if you're on a familiar basis with the person, which you probably are if you're calling them fat. Otherwise, use "vous êtes" (sounds like: voo zet). Use "gros" (sounds like: groh) if you're talking to a man and grosse (sounds like: gruss) if you're talking to a woman.