Is there an identification number on the axle which would let you figure out the size of the brake shoes? Dexter (if they made your axles) has a site that shows lots of info about their axles.
As an aside, you may find that ordering a complete backing plate would work out cheaper than individual parts. I believe that a complete plate includes springs, the shoes, and the magnet.
Will you be cleaning and re-greasing your bearings when you replace shoes? That sure would be a good time to do it, if you haven't done it recently.
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch