So to prod for bit more information, I am guessing you are running a 4K monitor right? Previously Fullscreen was the default mode of EU4 but we switched it to be Borderless which will make the game run at the resolution of the screen. It should be possible for you to switch to old normal Fullscreen and then set the resolution you want, restart the game and then it should be back to it's old self. Are you having problems doing that?