Backing plate vs shoe size, how to tell?

This is what we're all trying to explain to that poorhouse guy in another thread, but he's as thick as the Hoover Dam. So I just gave up. You have to match up the axle offsets with the offsets on the backing plates. Not to mention the diameter of the shoes, drums and backing plates. There's a lot more to it than just bolting some parts together, and he'll find that out the hard way.