At the rsk of being pedantic, I think the rear plate is better at 12 1/2 in by 7 in. This gives a better coverage of the edges of the rear light surrounds. It is in fact the width of the centre mounting plate. You are bang on George about the need to have it correct height to fit in the bracket and just be held by a couple of pieces of double sided adhesive foam strip, so NO SCREWS are used.
Now who is gonna be first to say they aint got the centre mount plate????