The more I dig into this, the more I learn (duh). Looks like I am definitely limited to 44x66 inches as my largest canvas for a custom size. Since the Z3200 doesn't have any large standard sizes, I am stuck. Time to put the print job on my iPF8100, I guess.
What doesn't make sense is that the printer driver allows you to click print, even though the job never gets sent to the spool. I didn't notice the small, blue warnings on the right hand side, and these have to be read and cleared somehow before the print job will actually go through.