You can drag with the mouse the rectangular area you want printed. The scale will be calculated from the shown area. Paper size will set the shape of the area, so if you drag a square with the mouse, the shape will be changed so that it is similar to that of the given paper size, but it will cover the area shown.
If the selected area is different than current paper orientation (Portrait/Landscape), paper orientation will be changed automatically. Paper size scale changes to a theoretical scale, which you can then adapt or print as it is.
You’ll find the function also in Drawing area -dialog.