The backing plates are basically 1" larger in diameter than the drum size. So, if you have 11" diameter backing plates, they're for 10" drums. If you have 12" diameter backing plates, you have 11" drums.
If there aren't shoes or drums on the backing plates, you can measure the backing plate offset to determine the shoe size, since the backing plates are not interchangeable. The method for measuring the offset and the measurements for a bunch of the different brake variations are listed in the link...
Converting rear drum brakes from SBP to BBP
