Qimage has the ability to make a print size based on you supplying *one* size of one side of the print. You will find that located in CUSTOM, Special Sizes, second item down.
Unless I misunderstand your question, that's not what you need.
You seem to need a way to make the driver know what size paper you are using.
You simply set the USER DEFINED page size in your driver, for 17 x 8.50, and let it print that size.
You can save that Printer setup in Qimage by clicking on FILE, SAVE, click the "P" button, and give your save a unique name; like, 17 x8 roll paper glossy.

Next time you want to make same size prints, just Click FILE, RECALL, "P" button, and select the 17 x 8 roll paper glossy, and all will be done for you.
You can make as many of these saved setups as you wish for various sizes.

Fred

Unfortunately for us all, the paper size is the job of the driver.
Once you have that selected, Qimage will sort the prints to use the allotted space of the page size you describe.

You should also try a few print placement settings, like Optimal Spaced, or Compact. There is also a SORT PRINTS menu iterm located in the RMB drop down menu when you Right click on the preview panel, upper right.

Fred

This is a print driver setting in the Canon printer properties control box.

See the attached file.

Tom

The good thing about using the Canon setting is that (1) It is adjustable (2) It does not waste an inch of paper and (3) It doesn't matter what print size you pick.

Tom