Oki C5550 MFP User Manual page 226

To create a new custom size:
(a) Enter its dimensions in the [Width] and [Length]
boxes.
(b) Enter a name for the new size in the [Name] box,
and click [Add] to save it in the list.
6. Click [OK] to accept your chosen size and close the
Custom Size window.
7. If this is the size of your document but you want to scale it
to fit a standard size of paper, click [Resize document to fit
printer page] in the Paper Feed Options window, and
choose the MFP's actual paper size from the [Conversion]
drop-down list.
C5550 MFP User's Guide
A guide to printing > 226