I can't change the resolution at all, the drop down is empty.
Try right clicking the executable and run the game as administrator (..\SteamApps\common\Divinity - Original Sin\Shipping\EoCApp.exe). That helped someone in Divinity 2 that had missing resolutions (though in that case it was only higher resolutions that were missing).
You can also try deleting/renaming the graphicSettings.lsx file (..\users\%username%\My Documents\Larian Studios\Divinity Original Sin), and see if getting the game to recreate it will fix the resolution options.
