Resize window during recording

I have a 21:9 screen and want to make videos for youtube. To do this, I need to shoot in 1920x1080 or 1280x720. Except that I never get the right resolution when I choose to save the window, and it gets really time-consuming trying to set the window size. If you could just make the window size automatically set to 1920x1080 (depending on the size you choose to preset), that would be perfect.