Are you wanting to make your background image or div the height of whatever window it’s being viewed in?
Of course, with anything CSS there are always multiple ways to create and or do something.
In this quick post I will talk about the latest and the greatest height:1vw;
1vh = 1% viewport height.
Now with that idea, we can fill the viewport with this.
100vh = 100% of viewport height.
Check out the results here JSFiddle example