You'll find all you need in here: "opera:config"
just type it in the address line. When it opens search "print"

Please: Where can I set the printer default page format ?? Currently defaults to "US Letter" so I have to change it every time I print.
There is no obvious setting in the preferences editor.
I've permanently set other packages such at Libre Office to A4 without a problem.
Using:
Opera 12.16
HP Photosmart 5510
Fedora
Gnome