Backing plate vs shoe size, how to tell?

Here's a listing for a 1970 Dodge Monaco with a 440 just as a example. It has 11x3" shoes. Other applications and engine/brake options might be different. What did the backing plates come off of?

http://www.napaonline.com/Catalog/C...hoes-Front-Proformer/_/R-RTSTS336B_0319298368