It's not possible because it would look weird if you just downsize them (they would be to small for the stairs then and not fit the steps). To fit into smaller planes, FSDreamteam would have to program complete new animations with crouching passengers. Umberto stated in another topic, they are thinking of trying to support small planes, but it will not be soon, because you need like another layer with a roof beside the floor so people crouch in between. So at the momet, there is nothing you can do about them heads sticking out of the plane.
