Size of x11 device

I've fairly recently changed my Linux desktop, and now use a XFree86 
server.  The default X11() window is rather small: it is supposed to be 
7" x 7".  However,

1) It is actually just over 5" x 5" and
2) 7" x 7" would be rather small on a 21" screen.

The reason for 1) is that DisplayWidthMM give 542mm, when it is really 
close to 400mm (and the correct monitor was specified).

So two questions.

a) does anyone know how to get the X server to give the correct size (on a
Matrox G550), or do we need (as on Windows) to have a means for the user
to override it?

b) should there be an option to set the default size of an X11 device?
(On Windows the default size depends on the screen size, since 7" would be
very large for a 10.4" LCD.)

The answers probably depend on how common a problem this is.

Brian

Brian D. Ripley,                  ripley@stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595