Apple has long supported CUPS which allows you to define paper sizes for print.CUPS
CUPS is the standards-based, open source printing system developed by Apple Inc. <http://www.apple.com/> for Mac OS^® X and other UNIX^® -like operating systems.
http://www.cups.org/index.php
I am a Linux Ubuntu user, not Apple. You will have to pay particular attention to any Apple OS versions.
I have done it for HP PhotoSmart Pro B9100 printer. It is not painless.
If I recall, you will have to find your printers PostScript Printer Description ("PPD") file.
_Make backup files first of anything you might change._
In several locations, there will be definition areas. You will have to create those definitions and
then save the file.
I spent several hours getting comfortable with the material involved through Google.
Then copied and renamed and adjusted PPD file based on existing definitions.
Copying a definition closest to the size you want will definitely help, assuming it works the same.
Caveat: If the printer is not physically able, this will be of no use.
Good luck.
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/** <https://bugs.launchpad.net/bugs/213984>