Hmm, I'll look at that and fix it. There is a config file that you can change the settings in. It will be C:\Program Files\Kivi's Underworld Demo\User\user.cfg in Windows or /Users/YOUR USER NAME/Library/Application Support/Kivis Underworld/User.cfg on the Mac.
Inside that file, you want to change r_height and r_width to whatever resolution you want to run in.
Depths of Peril, Kivi's Underworld, Din's Curse, Drox Operative, Zombasite, & Din's Legacy