WP introduced a new filter called admin_post_thumbnail_size for you to be able to change the default image size in the admin panel.
To change the size of the featured thumbnail image you can use something like this:
function custom_admin_thumb_size($thumb_size){
return "banner-xs"; //or use something like array(400,400).
}
add_filter( 'admin_post_thumbnail_size', 'custom_admin_thumb_size');