Please read this article first, if that did not help, proceed with the steps below.
- Right click steamapps/common/crusader kings 2/ck2game.exe
- set compatibility to run in 640 X 480,
- then start from the edited .exe.
- Adjust your resolution through the in-game options
- Exit the game
- Remove the check from 640 X 480 box
- Start the game from the steam library