User`s manual

50 Printing from a Mac
3A. Create a Custom Page Size (Optional). If the paper size you
need to print to is not already in the drop down menu options
select Manage Custom Sizes.
3B. Add a New Custom Paper Size. Click the plus (+) button to add
a new custom paper size. Enter the size on the right. Be sure to
adjust the top, bottom, left and right margins to 0. Change the
name from "untitled" to the size of your custom page (W x H).
When you have finished adding your new custom page size,
this size will appear in your drop down menu options for paper
size on future print jobs.