The current rawhide a2ps package has the default medium set to "_glibc", which is apparently some magic string to tell a2ps to ask glibc what paper size to use by default. The first problem is that this isn't documented anywhere in the a2ps documentation, as far as I could tell. The second problem is that the functionality in glibc depends on the LANG variable being set properly, and LANG does not get set properly in shells started for cron jobs, because /etc/bashrc only executes the stuff in /etc/profile.d in interactive shells. As a result of this, cron jobs which call a2ps (and I have one, which is why I noticed this) use the wrong paper size. I'm not sure the bug is in a2ps. In fact, the bug may be in the decision not to set i18n variables properly in non-interactive shells. But it manifests itself in a2ps, so I'm filing it under that component. If y'all end up deciding that the problem is really with /etc/bashrc, you can move the bug into the "component" category or something.
I think that the second problem is a symptom of the first; it should be documented that LANG needs to be set correctly for automatic paper size to work, and that it may not be in non-interactive shells.
Documentation added in a2ps-4.13b-18.