First, thanks for the nice comment and glad to hear it’s coming together for you.
Regarding the slider, The position is responsive so it will change the size of images with a relative sizing method. For example, if an image is 800×400 and it goes into a space that is 400px wide, the height will be relative to the width…in other words, this would be 50% (400×200).
The overall width for this space is maxed at 1920px wide because this theme is designed to stretch full width of whatever the person is viewing the site in. If possible I would recommend your images being 1920px wide for this theme and tall enough so that when it shrinks due to the responsive sizing, that the height will look good when smaller.
Hope that helps?