In most cases the default resolution set is the best your hardware can support. If you need to change the default resolution please edit the file "/etc/X11/xdm/Xsetup" and uncomment the appropriate line.
# to set the resolution to the highest supported,
# uncomment the line that reads
# xrandr -s 0
# to select the resolution 1024 by 768,
# uncomment the line that reads
# xrandr -s 1024x768
# to select the resolution 1280 by 1024,
# uncomment the line that reads
# xrandr -s 1280x1024
For specific graphics chipsets, like the VIA, the X server does not recognize support for resolutions above 800x600 on a thin-client. In such cases, it helps to add the following entries to the section "Monitor" in the file /etc/X11/xorg.conf before restarting X.
HorizSync 30-90
VertRefresh 48-120