Hmm....
Go to your Documents folder C:\Users\<Name>\Documents\Paradox Interactive\Crusader Kings II
and delete the settings.txt file.....or open it and look if the resolution (x and y values) fits you. Change accordingly.
If that didn't work or the resolution in the file is fine, then go to your games folder,
right click ck2game.exe
Properties - Compatibility tab
and check "Disable scaling on high DPI settings"
I suspect the first.
Simply deleting the settings.txt file should do.
It recreates itself on start and should have the right values then.