This might help: Page 19 - 20 in your manual 'Printing from Start to Finish'
Select the paper Size setting for the paper you loaded. If you loaded roll paper or canvas media, create a custom paper size. Select User Defined in the Size menu, then click the User Defined button. Type a name for your custom size. For the Paper Width, enter 8.3for A4- or 12.95 for Super B-size rolls. For the Paper Height enter the desired length up to 44 inches. Click Save, click OK, then select your custom paper size in the Size menu.