(Mainly applies to non-Wasp printers without a User-Defined setting available in page setup)

The list of paper sizes you see there are put in place by Windows.  If you are using a custom label size that Windows doesn't have in place by default you can create a custom form by going to Printers in the Control Panel, clicking on File, then Server settings.  You can create a custom form on this screen that will then be available for all Windows printers, including the Wasp printer.