I suspect that English is not your first language, so it's not completely clear to me what you're asking. What you have said here is that you want to describe

I suspect that English is not your first language, so it's not completely clear to me what you're asking. What you have said here is that you want to describe thing X to person A, and person A is fat. I suspect that is not what you mean. Are you saying that you want to describe person A to person B, and person A is fat? If I'm right, and English is not your first language, perhaps you could ask your question in both English and your native language so we can cross-check, and then possibly answer.