There is a green custom image icon to the right of the list field that’s I want to be 40x40px, (the size shown in the screenshot.)
I am able to override the styles and increase from 16px to 40px in Dev tools but when I add the CSS highlighted to the stylesheet, the change shown in the screenshot does not update.
The 16px in all the places are forcing this icon to 16px. I recommend finding that CSS and editing it to be 40px, because overriding it with the !important is going to be difficult. Try editing that to be 40px for the background size and the width and height, and let us know how that goes.