When you first open Messenger, you see the word "Chats" at the top of the screen. Then you choose someone to video-chat with and the screen opens up to a chat window. At the top on the right are 2 icons, one for a "phone" call and one for a "video" call. If you choose "video" the session will start, with you calling the other party. Once they answer, you see 2 live video windows: the main one, with your party visible, and the smaller one, which shows you.
I am asking about whether that window that shows you, the smaller, embedded one, can be sized even smaller...it's too big now for my taste. As I said, on Facetime, you don't have this issue as that embedded window is smaller.