I am not by my printer / computer, but there is an option in the Canon plugin that either trims your print to the full size of the sheet, or to the size of the image. I believe it's under a Roll Settings button and is called "Trim to image" or something to that effect.
If you've chosen to print a 6x9 image using a vertical 8.5x11 sheet, onto a 24" roll, selecting this option will cut the roll in and around 9" (plus minimum margins) rather than at 11".
Terry.