"Someone told me I'm fat"

Thank you!

If somebody told you you look fat in general, you'd probably say "Someone told me I look fat." If somebody told you you looked fat on a certain day or in a specific dress or something, you'd probably say "Someone told me I looked fat (in that dress)." But I don't think most people would notice the difference.

Depends on the intentions of your sentence. You would need more context to choose the correct one. "Someone told me I look fat today" "Someone told me I looked fat in that dress". Both can be correct, but it entirely on the tense you are hoping to utilize.