On Windows the return values of `display-mm-width' and
`display-mm-height' don't really reflect the screen width and height
in millimeters.  I attached a patch which yields more realistic
results.

In my case:

                   display-mm-*   display-mm-*
        Real size  without patch  with patch
Width   285mm      370            279
Height  215mm      277            203

For a preview-latex user who originally reported the problem:

                   display-mm-*   display-mm-*
        Real size  without patch  with patch
Width   330mm      320            305
Height  207mm      240            203

The original report described that images generated by preview-latex
(which uses `display-mm-{width,height}' for that matter) are distorted
horizontally (see bug report at
<URL:http://permalink.gmane.org/gmane.emacs.auctex.bugs/673>).  In my
case these images were too small compared to the font size (see bug
report with screenshot at
<URL:http://permalink.gmane.org/gmane.emacs.auctex.bugs/449>).  With
the patch applied both of these problems get better.

-- 
Ralf