[CentOS] Changing default paper size to A4 on CentOS 5 (Kyocera FS1920 printer)

Hello,

I have a Kyocera FS1920 network printer, and I run an up-to-date
CentOS 5.6 x86_64 workstation.

When I try to print with evince or Firefox the default paper size is
always set to US Letter whereas the printer use A4.

I have searched and other people seem to have the problem, but the
only recommendations that I have found boil down to setting the
default paper size to A4 in the CUPS printer settings.
I had done that already and double-checked via the Printer config UI
or the CUPS web interface (http://localhost:631), but to no avail.

What is a bit weird is that I do not remember having such problems
with my previous printer (an Epson Stylus RX500).

I would be grateful for any hint / idea.

Cheers,

Mathieu