Canon LBP6780x Manual page 161

Make sure that the paper stack does not exceed the load limit mark lines (A) or (B).
(A): Paper larger than A5
(B): Paper smaller than A5
Be sure that the paper stack does not exceed the load limit mark lines. If the paper stack exceeds the load limit mark
lines, this may result in misfeeds.
6
Set the paper drawer in the printer.
Next, register a paper size.
"Registering the Paper Size"