In Mac OS X, the default paper size is a setting that determines the size of paper that applications will try to print to unless the user chooses otherwise. For most systems and applications this defaults to US Letter. In order to provide a positive user experience, especially for end users outside of the United States, it may be desirable to set a different default paper size.
The file that contains this preference is:
This makes the preference domain: com.apple.print.PrintingPrefs
The Key that governs the preference is called : DefaultPaperID
DefaultPaperID contains a string value that corresponds to a paper size. See the chart below:
Paper Size String
US Legal na-legal
US Letter na-letter
JIS B5 jis-b5
Envelope #10 na-number-10-envelope
Envelope DL iso-designated-long-envelope
Tabloid Oversize arch-b
ROC 16K roc16k
Envelope Choukei 3 cho-3-envelope
Super B/A3 arch-b-extra
To set this preference with a script use the command below, replacing <paperSize> with the correct string for the intended paper size.
For a single user, run as the user:
defaults write ~/Library/Preferences/com.apple.print.PrintingPrefs DefaultPaperID <paperSize>
For all users, run as root:
defaults write /Library/Preferences/com.apple.print.PrintingPrefs DefaultPaperID <paperSize>
This process has been tested on Mac OS X v10.6 (Snow Leopard), v10.7 (Lion) and v10.8 (Mountain Lion).
I hope you find this useful.