Oki C 5400n - Manual (Page 197)

Windows: PCL Emulation Driver • 195
5. On the Setup tab, under Media , select User Defined Size in the
Size drop-down list. Define the custom size in the dialog box which
appears, then click OK .
NOTE
You may see a Warning dialog box indicating a
conflict. If you do, click OK and the driver will
automatically make the needed changes to correct for
the conflict.