Hello Getch,
You can register a custom paper size for the printer using the steps in the link HERE. I checked, and magnet sheets did not show up in the list of supported paper types. There is a chance that it will not work, but you can try loading the paper into the multipurpose tray.
For further assistance with any driver settings, please let us know what the specific operating system is running on your computer and we can provide some suggestions. You can also contact support at 1-800-652-2666. Our agents would be happy to help.
