Xerox Phaser 6120 Reference Manual page 84

To delete an existing custom paper size, select the custom paper
size from the list on the center of the dialog box, and then click
Delete.
5 The custom paper size setting is saved, and the name of the custom
paper size can now be selected from the Paper pop-up menu.
6 After you have finished entering the settings, click Done.
Specifying Page Setup Settings
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