The Nelsonic Space Attacker size (which seems to be common across that, the Armitron/Waltham/Nelsonic Dixie watches, and Sexum & variants). Just a bit bigger than most other watches of the same vintage.
The smaller ones are reasonably easy to find, but this particular shape/size is not (well, not for me anyway) - much of the ones I turn up that look like they might work end up being glued to the back of the polariser/LCD, as Kasper mentioned.
Someone has to be making them still - a reflector mirror sheet about A4 size would last me pretty much forever, but trying to find one leads to a lot of nope.
This stuff doesn't really look the right colour for a G&W so suspect it wouldn't give a great end result on watches either —
http://www.ebay.com/itm/NINTENDO-GAME-W ... 1848303434