I think I found the solution to this, you just have to add the paper settings to the printer in order for the page setup dialogue to show the option. It’s pretty easy to do, but the precise procedure may differ depending upon the printer driver. For mine, once I’m in the Preview screen:
- Click the printer driver I wish to use.
- Click the “Preferences” button (which is confusingly next to “Print to file”, but impacts all print settings nevertheless).
- Click on the “Paper/Quality” tab.
- Here you should see a “Custom” button in the paper options section, click that and setup a 6x9 paper size. As the help text indicates in this dialogue, you will need to reload the Preferences tab to see it in the list. However, we’re not worried about changing that here.
- Click OK to dismiss the printer settings dialogue, then Apply and then Cancel.
- Now you should be able to select 6x9 from the page setup button (the second from the right), and choose your new 6x9 setting.
I don’t actually have a printer hooked up to this computer, so I can’t test it further, but I’d be interested to see if it actually tries to print out at 6x9 when going from there, or if additional steps are required. It sounds like all you need is a preview though, so this should be good enough for that at least.