HP Color LaserJet Enterprise M751n Laser Printer - Manual (Page 45)

8. The tray configuration message displays on the
printer control panel.
9. Press the OK button to accept the detected type
and size, or select Modify to choose a different
paper size or type.
For custom-size paper, specify the
X and Y
dimensions for the paper when the prompt
displays on the printer control panel.
X
Y
ENWW Load paper to the 550-sheet paper tray 35