1999 GMC Yukon brake help

[RalphieN]

I'm not sure I'm posting in the right area. Forgive me if so.

I dont know a whole lot about cars but my boyfriend is having some issues with the back brakes on his Yukon. I believe they are drums. But he looked at them tonight & it seems that a big chunk of one of his brake pads has broken off. He says they keep losing up or something. Like they have a self adjuster thing on it that is supposed to tighten up as the brakes wear down. But it doesn't seem to be adjusting. Just getting looser. Would anyone have any clue why this would be happening? Could it be the wrong size on there?? 

Auto Zone says either 10 in or 11 in. 11 in was what was on there b4 he changed them, but could that have been wrong? Maybe previous owner had wrong ones on there? 

Sorry if this is confusing. Any help would be appreciated.

It's a 1999 Yukon

[Supreme Pizza]

What caused him to inspect the rear brakes to begin with? Noise? Poor stopping power?