Right click the '..\SteamApps\common\Divinity Original Sin Enhanced Edition\Shipping\EoCApp.exe' program file, select Properties, switch to the Compatibility tab and check off the option to disable display scaling in high DPI settings, and maybe run as administrator.
Check your driver settings for any resolution scaling options (right click an open area of the desktop to start the nvidia Control Panel).
Windows 8 has a global scaling option, as well.
Try browsing to the '..\Documents\Larian Studios' folder and deleting the 'Divinity Original Sin Enhanced Edition' subfolder.
This folder contains the saved games, configuration files and a level cache folder. Normally I would suggest renaming the folder for troubleshooting purposes, but if you don't have any saved games that isn't an issue.
Next, extract the replacement folder from the download below into your '..\Documents\Larian Studios' folder, and see if that will let you start the game and see the full screen.
https://www.dropbox.com/s/1ipz4q9780x35tj/DOSEE_doc_20119430.zip?dl=0The graphicSettings.lsx file is set to 1280x720 Windowed mode, Very Low quality preset, which you can change in the options (manually, or hit autodetect) if this gets the game to start.
If that doesn't help, delete the downloaded My Documents D:OS folder.
