I have much of the same thing where basically I want my resulting PDF to be whatever the canvas size is. I’ve played with the Canvas settings, different “print to”, made a custom paper size, set Distiller’s default page size to what I need, but no matter what it seems if the drawing started out in 8.5 x 11, that is always the PDF size.
PageSize; PDF size; CanvasSize, all worked perfectly in OG4. And I mean perfectly, OG4 allowed varying Canvas Size and magnification in a single document. Omni broke it, badly, in OG5. They have not heard about backward compatibility, and how to retain customers. Each new development team appears to start from scratch, and ponder, gee, what does the latest tablet market need, without any reference to the specific capabilities of the previous version, the one that made the company great.
OG6 is a dog. More like a hush puppy that hasn’t been weaned.
So you have to fiddle and do an Irish jig.
Start with a new doc. First, via File/PageSetup set the paper size. 11 x 17 or 17 x 22 or whatever. That should be the CanvasSize. The Rulers, etc should be anchored on this size. This is once, for the entire document. (Unlike OG4, which allowed Scale on each Canvas.)
In the same window, set the Scale. Eg. 80% of a 11 x 17 would result in a CanvasSize of 13.75 x 21.25. When that is printed (directly or via a PDF), it will of course be reduced to 11 x 17.
If that does not work, lodge a support request via Help/Send Feedback. Then wait three or four years.
Separately, you can fiddle with some setting on each Canvas, via the Inspector/Canvas/Size. You need to be in the mood for dancing. A crystal ball helps, in order to figure out what the developer was thinking, before he learned what “intuitive” meant.