Windows XP:

Start, Printers and Faxes
File menu, Server Properties.
In the middle of the form is an option called "Create a new form", check this box and then enter your dimensions below. Then click the "Save Form" button to add this size to the list.

Windows 7:

Start, Devices and Printers.
Click a printer.
In the toolbar, click Print server properties.
In the middle of the form is an option called "Create a new form", check this box and then enter your dimensions below. Then click the "Save Form" button to add this size to the list.