Right click on the FM07 demo shortcut on your desktop. Go to 'properites'.
Find the'target' box.
An example...
My target box has this in:
<pre class="ip-ubbcode-code-pre"> "C

Program Files\Sports Interactive\Football Manager 2007 Gold Demo\fm.exe" </pre>
I want it to display properly on my widescreen monitor in fullscreen mode. I know that my monitor runs at 1920x1200 resolution. Therefore I modify the target line to read:
<pre class="ip-ubbcode-code-pre"> "C

Program Files\Sports Interactive\Football Manager 2007 Gold Demo\fm.exe" --fullscreen_height=[1200] --fullscreen_width=[1920] </pre>
Obviously your widescreen monitor might run at a different resolution so change the numbers accordingly.