Translations:Xorg/Guide/65/en
From Gentoo Wiki
Now change the resolution. In the next example from /etc/X11/xorg.conf.d/40-monitor.conf we add the PreferredMode line so that the X server starts at 1440x900 by default. The Option in the Device section must match the name of the monitor (DVI-0), which can be obtained by running xrandr. Install xrandr (emerge xrandr) just long enough to get this information. The argument after the monitor name (in the Device section) must match the Identifier in the Monitor section.