Make a windowmanager/desktop (Gnome) use only a part of an X11 server’s (Xvfb) resolution

Take a look at devil's pie, an app written to allow such customization on GNOME 2.

Note that if/when you move to GNOME 3, things might work very differently…

There is an easier way to do this if you are going to go headless only: Don't use a standard X server.

I do this fairly often on boxes that almost never have monitors attached. This example uses xdm on debian, but I have done it with kdm and gdm.

specify Xvnc as the X server to use with its full path and specify the resolution on the command line. for example here is one of my /etc/X11/xdm/Xservers from a headless box.

:0 local /usr/bin/Xvnc :0 -nolisten tcp -desktop crystal:0 -geometry 1400x1050 -rfbauth /home/ben/.vnc/passwd

and just to make things nicer I created /etc/X11/Xsession-0

#!/bin/sh
#

vncconfig -nowin &
# invoke global X session script
. /etc/X11/Xsession

and add to /etc/X11/xdm/xdm-config the following line

DisplayManager._0.session:      /etc/X11/Xsession-0

to automatically start vncconfig on (and only on) all known Xvnc instances. On mixed local remote setups I use two X servers, one traditional one virtual. works like a charm. this unfortunately is more difficult on the current ultra user-friendly display managers because they try too hard to make things easy.