When cardstock or an envelope is loaded in the Multi Purpose Tray, set the paper size and type and then register the paper size on the Basic tab of the print settings screen in the printer driver. Once registered, the paper size can be selected from the Page size menu.
Display the print settings screen
Select the Basic tab
Select Page Sizes...
Select New
Enter a name of the paper
Enter the paper size
Select Save
Select OK
In the Print size menu, select the page size (name)
Select MP Tray in the Source menu
Select the type of paper in the Media type menu