Hi karoooszka
Go into Control Panel > Devices and Printers, right-click the printer and go to Printer Properties
Select the Configuration tab and go into Custom Paper Size. Whatever you put will then become available in the driver.
I'm looking to add/change the custom paper size for my Xerox Workcenter 6027. All 20 of the custom paper sizes are set for 8.27 x 11.69 with no button/option to change them. I'm running Windows 8. The drivers are up to date and the firmware on the printer is updated to the latest version.