In Integral Options > Welcome section.
See here in this screenshot https://www.dropbox.com/s/rohflowpfyzrlz4/welcome-background-settings.PNG?dl=0
The height of the Welcome section depends on how much text you have in the title line 1, title line 2 and tagline.
The background image (on mobile devices) will take up 100% of the height of the window.
That is how it should be.
If you set the background image to be 100% of the window, on small screens there will be empty space which the background image doesn’t cover.