Print: Match document size against known paper sizes
Custom paper sizes work with some printer drivers but not others. EPSON's ESC/P-R CUPS driver for example scales a custom-sized document to ISO A4. (It apparently used to just bail on unknown page sizes.) Matching the document size against a known paper size in Inkscape prevents the driver from scaling to the wrong paper size. Fixes an issue discussed in comments 3, 4, 5, and 6 of <https://bugs.launchpad.net/inkscape/+bug/630635>.