Mac OS X pstopdf utility defaults to Letter page size
Solution 1:
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.