Hi Curtis. Sorry for the late response. Default size of an image is defined only in custom-header.php. CSS (.header-background-image class) controls it only in a way that will stretch it full size based on width (screen size), and height of the header is controlled by the amount of content in Site title and site description and site-branding class (with the min-height set) so if the the screen is larger than default size it will zoom it to fit the screen and the bottom of the header image might get cropped.
Unfortunately i don’t have flawless workaround around this issue at the moment.
