Have you actually measured the size of the print?  Some papers are a little more than 24 inches wide so it may actually be printing 24 inches.  Also, you have "Fit roll paper width" selected and while the roll width is the same as the document width, there will be scaling with that option, particularly since your PPI shows 606 on Qimage Ultimate's preview window.  That driver seems to have some non-standard settings when it comes to borderless printing.  In addition, when you UNcheck borderless, you still get the full width (24 inches) so I'd suggest UNchecking borderless and be sure not to check "enlarged/reduced printing".  If you UNcheck both borderless and "enlarged/reduced", the driver should produce what is sent to it instead of modifying it.  Qimage Ultimate is sending exactly 24 inches to the driver but unfortunately you are at the mercy of the driver after that: you have to make sure your driver settings are set to produce what it gets without modifying it.

Keep in mind that trying to print exactly 24 inches on a 24 inch roll may prove nearly impossible without getting at least some paper loading error which will result in a small sliver not being printed on either the left or right.  Nothing you can do about that since it doesn't appear that this printer can do overspray on a 24 inch piece of paper.

Mike

Yes, it has to be driver related.  The queue shows 24x24 so we know QU is sending 24x24 to the printer.  If you get less than that, then the driver is doing scaling.  Did you try turning borderless off?  You still get the same paper size so I'm not really sure what borderless is doing in this driver.  The driver seems to have a mind of its own too.  If you set everything up the way you did, you get 606 x 606 PPI which means the driver is doing some type of scaling.  If you simply turn on the print preview by checking the print preview box on the main tab, PPI goes to 600 x 600.  Turn it off: back to 606 x 606.  I really have no idea why any driver would do that.  A preview should not be affecting how the driver works: it should just give you a preview.  Either this driver is buggy or Canon is doing some very strange (non-standard) things in that driver.

Mike