Hm. Not sure about that one. Traditionally it has been a command line tweak fix. From the official MS support page:
Change the default resolution on widescreen monitors
If you have a widescreen monitor, you can modify the default resolution. Create a desktop shortcut to your game's .exe file, and then modify the command line in the shortcut by adding the arguments "width=xxxxx height=xxxxx" to the end of the command line. To do this, right-click the shortcut on your desktop, and then click Properties. In the Target text box, add the following text to the end of the existing path after the quotation marks, width=xxxxx height=xxxxx--for example:
"C:Program FilesMicrosoft GamesDungeon Siege 2DungeonSiege2.exe" width=1680 height=1050
Off-center Character Portrait on widescreen monitors
When you play on widescreen aspect ratio (1920 x 1200), the Character Portrait will be off-center.
Official link:
http://support.microsoft.com/?scid=kb;en-us;902423