bigger rear drums

Not to hijack this thread, but had a question....On the topic of rear drums. How are the rear brakes measured anyway to determine the size? (9, 10 or 11") Sounds like a stupid question but I have a couple pairs of backing plates only that are bare. Do you just measure the diameter of the backing plate? What measurment do you actually take to determine what you have?.....Sorry bout' this. Thanks.

:book: