fikmaken, try this..
With the game loaded, press the tilde ` to open the console, then type -
setres 1920x1080
Or, you can do this -
Go to C:\Program Files\Steam\Steamapps\common\Red Orchestra\DarkestHour\System\DarkestHour.ini
Double click the ini file, it should open with notepad, then search for [WinDrv.WindowsClient] and under this line you should see:
FullscreenViewportX=1920
FullscreenViewportY=1080
set these values to what works for you.