As far as I can tell, changing the default paper size currently requires editing either groff font configuration or (if it's set up that way) a system configuration file such as /etc/papersize.  Systems that use libpaper allow some more flexible approaches: you can use the LC_PAPER environment variable to use a default paper size based on the locale, or the PAPERSIZE environment variable to set it directly.  Would it make sense for groff to have optional support for libpaper?  Looking at paper.h, its API doesn't seem outrageously complex.

Of course, one can always use something like `-P-pa4`, so this isn't strictly required, but it would allow somewhat better integration with systems that support libpaper: groff would then use the same approach for selecting the default output paper size as other programs.

(This bug report was inspired by https://bugs.debian.org/1091375 and https://bugs.debian.org/1091376.)