header.site-header, img.size-full, img.size-large, img.header-image, img.wp-post-image {
    max-width: 2000px;
    height: auto;
    width: 100%;
}

I don’t know why your header code didn’t have the desired effect, this is just a cheap workaround. The ultimate size is still constrained by the page container that it’s within. If you want it to be wider than this, the template HTML really needs to be restructured. You could instead try playing with negative margin CSS to get it to overrun the page container, but it’s not a very good solution.

Be sure whatever you end up with still works on smaller screen sizes. More CSS within media queries may be needed.