I'm having difficulty adding a custom label definition. I'm able to do the label layout, but not able to get the custom paper size. I'm working with four-up labels on a 6"x12" sheet. I can not find where/how to add a paper size definition that will appear in the custom label dialog under page size. At the document level, I can see my custom page size listed (as defined at the printer level), but when I get into envelopes and labels, only specific label-related page sizes are listed, not the ones I've defined.

How/where do I create a custom page/paper definition that will be available when I go to create a custom label format? I'm trying to set up our Firm's standard shipping labels so they can be used with mail merge instead of a custom document template I've distributed as an interim step.