Winter Overcoat

Hello! Does any of you know how an overcoat should fit, or how to try it on at the store (with a suit jacket on…maybe? I’ve read that the way they’re made (according with the corresponding suit jacket or sports jacket size)allows enough room to wear it on top without feeling tight or overwhelmed. However, I still don’t want to look square, I kind of want the overcoat to give me a little shape; I’m sort of a fit guy. Your thoughts, please! Thank you!