How Do i Decrease Dawn Theme Image Banner Height

[dam003]

Please I need help with reducing the Image banner Height on Dawn theme for desktop website view.

I designed the banner on canva and initially used 4320px X 2160px, then tried reducing it to 1200px X 600px and it was still the same height .

I use the "adapt to first image" when customizing the banner height in dawn theme, because when i use "Small" the sides of all my banners get cut off in the mobile view.

I figured out how to reduce the banner height for the small banner to the size i want on the desktop by changing the code  in section-image-banner.css 

.banner--small:not(.banner--adapt) {

   height: 25rem;

}

25rem is the perfect height for me on the desktop view, but like i said i cant use the "small" option, because the width of the banner image doesn't show fully on the mobile view.

How do i adjust the Image Banner height for the desktop when i use the "Adapt to first Image" setting?
Any help will be highly appreciated.