Large format printers will not do borderless printing on sheets. If you want this option, you must print on a roll and you can than put the settings to do borderless and your output size. If the roll is 20", you can do your 20x24 so it does the 20" end on the width and 24" on the length; otherwise use a 24" roll and put the mode in the correct way to fit it properly.
This option is the same for both Mac and PC, but borderless printing only with rolls unfortunately if using a printer 24" or larger in the aqueous models.
Per Canon's site for the Pro-2000/4000:
Borderless Printing Width
10, B4, A3, 14, A2, 17, B2 (20.28/515 mm), A1 (23.38/594mm), 24
Hope this helps.