I don't know how to respond anymore

I've been getting called "Fat" or "Thick" for the past 2 years by my siblings or friends, and i don't know what to say anymore. Whenever they called me fat 2 years ago i never used to think that much of it but now more people have clinged on and started joining in with the fat calling. It's made me seriously upset and I've got to the stage of avoiding food for the past week. I'm normally quite good at standing up for myself agaisnt people but I can't help but avoiding looking at myself in the mirror. I'm 17 and I'm just below average in weight.. But I really need help on what to say back in response.