[CentOS] CUPS issues

We've got this huge HP DesignJet Z9 printer, for printing posters.
Except... I got the latest hplips installed, it knows this printer... but
there are no poster-sized settings.

Now, I've started hacking the .ppd, as I did six year ago for the old
poster printer, but it seems, after a good bit of googling, that it would
be easier, when I hit print from okular, and get the dialog window, to go
to properties->page, and set it to custom, and set the width and height...
except I can't: both width and height are grayed out.

Now, I'm running as me. For reasons I don't need to get into, I shouldn't
be doing this as root.

Anyone know what I need to modify to all me to set this? Hacking the .ppd
is half a day or a day of tedious work.

      mark