ok, re-look at your post and I think this is what you want. (As krikor posted)
Use the crop tool, with "delete cropped pixels" / "fixed aspect ratio" / aspected ratio set to 16:20
Draw in the crop and position.
Click in the canvas to crop
Resize to suit. Image -> Scale Image with width / height linked (should be default)
Use the crop tool, with "delete cropped pixels" / "fixed aspect ratio" / aspected ratio set to 16:20
Draw in the crop and position.
Click in the canvas to crop
Resize to suit. Image -> Scale Image with width / height linked (should be default)