Use the UniWS
program to set your resolution, You don't have to use another EXE as the program says.
Go into game and select your custom resolution.
The following two fixes were found at GoGWiki
Because of the resolution change, your UI will most likely be (partially) invisible.
Open your "Hardware.ini" file (steamapps\common\prince...) and change:
===========================================================Fog bug (or: "OMG I CAN'T SEE A DAMN THING!!")
If you can barely see anything on the main menu background or in-game like I experienced, change these values in your "Hardware.ini":
Set your "Hardware.ini" file to read-only, just in case the game decides to write to it. (it didn't in my case)
Hope this helps :-)