Well, the updated configuration tool seems to be a bit picky indeed, but 1680*1050 is selectable, so it's not aspect ratio thing only.
The configtool asks for possible resolutions to the graphics card / monitor through Windows, but it could indeed be that you want to play (and will be able to play) in a resolution that is not listed. We are just showing the "safe" resolutions that are trusted / sure to work.
If you want to try other resolutions, you can indeed manually edit the config.div file (in the root of the installation directory) with a text editor.
