I have mine set to the following for full screen. It works just as well on a 27” display as on an old iphone 4, and everything in-between.
There are a maddening amount of options, but here are the crucial ones.
(Edit Slide)
Slider Main Image/Background
Background Source: Image BG checked
Background Settings:
Fit: cover
Repeat: no-repeat
Position: center top
(Main Slider Settings)
FullScreen Align: on
Force Full Width: on