Цитата man xorg.conf:
.............
MONITOR SECTION
The config file may have multiple Monitor sections. There should normally be
at least one, for the monitor being used, but a default configuration will be
created when one isn't specified.
Monitor sections have the following format:
Section "Monitor"
Identifier "name"
entries
...
EndSection
.................
DisplaySize width height
This optional entry gives the width and height, in millimetres, of the
picture area of the monitor. If given this is used to calculate the hor-
izontal and vertical pitch (DPI) of the screen.
................
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Насколько я помню - этот ключ уже давно существует, чуть-ли не с XFree86 4.0 а может и раньше
http://lists.altlinux.org/pipermail/...ay/122300.html