You can change the resolution by opening the graphicoptions.xml file in Notepad, or other text editor, and changing the values for ScreenWidth and ScreenHeight.
In XP this file is in the folder
C:\Documents and Settings\ %username% \Local Settings\Application Data\Divinity 2\Profile
or in Vista / Win 7
C:/Users/ %usename% /AppData/Local/Divinity 2/Profile