Format for a4 paper

Hello.

How can one choose a specific paper size when
formatting docbook documents for printing with
the docbook-utils applications, like docbook2ps
and docbook2pdf? Is there a command line option
for this? Or a configuration file to be edited?

If this options is not available with the tools
in docbook-utils, are there any other tools that
would give this functionality?

I am running Gentoo Linux and I am not yet a
document writer using docbook.

Romildo