HP Color LaserJet Enterprise CM4540 MFP series - Manual (Page 115)

4. Close the tray.
5. The control panel prompts you to set the paper
size and type. Select the Custom setting, and
then configure the X and Y dimensions of the
custom paper size.
NOTE: See the label in the paper tray, or
the following illustration, to determine the X
and Y dimensions.
Y
X
ENWW Load paper trays 95