Canon LBP7210Cdn Manual page 271

Display the [Custom Paper Size Settings] dialog box.
(1) Display the [Page Setup] sheet.
(2) Click [Custom Paper Size].
Specify the custom paper size.
[Name of Custom Paper
[Unit]:
[Paper Size]:
Size]: Enter the name of the custom paper size to be added. Up to
31 characters can be entered.
Select the unit to be used when setting the custom paper size.
Specify the height and width of the custom paper size
([Height] [Width]).
Specify the custom paper size in portrait orientation
([Height] [Width]) within user-definable sizes.