Mac OS X pstopdf utility defaults to Letter page size
If I remove all page size information from a PostScript file, then Preview renders it on a Letter page, even though my default Paper size in CUPS is A4.
There are no user-accessible options or preferences that I can discern. It's certainly undocumented, if it exists.
You may be better off trying to explicitly define the page size in a2ps.
Alternative PDF creators, like GhostScript, offer greater control over the output.