look what i just found !

You do'nt want the backing plate to be as wide as the pad itself. You want a little extra pad all around. Larger diameter for the pad than the backing plate. If you bump the edge of the pad against something, it will be the pad and not the backing plate, so it won't cause any damage. Example: if you're using 6" pads use a 5" backing plate.

Thelink I posted above for the plates will tell you what size pads are good to use for the size backing plate you choose.