I’ll tell you. When it’s the size of a matchbook. This is the kind of mouse a spy might use — if he or she was unaquainted with the magic of trackpads.
Really, all you need to make a mouse is an optical sensor and a couple contact sensors for buttons, and they can make those on a molecular scale these days. I wouldn’t be surprised if these mousemakers (I’m guessing Vietnamese by the music) get one-upped by some puckish nanomaterials engineers who etch a functional mouse on the side of a human hair.