If I create a postscript document from some documentation, such as the
PMake manual, it seems to be set to a page size slightly larger than A4
by default (letter?).

How can I change the page size that the output is typeset to?  I assume
this is a groff thing, and not fixed in the original document, but I
can't find any documentation on this, and I don't know much about the
roff family of commands.

Neil.