Edit box-header on Wordpress Dashboard

Question

In my theme, I'm working with two featured images. One in square shape (preferably 500*500) and one in widescreen (1920*1080 for example).

However, it's not clear where which image has to go, when I make a new post, so I am looking into changing the title element of a box on the dashboard, so it's clear which image goes where.

See attached screenshot. I would like to add 500px*500px after Featured Image, but I can't find out where I do this.

I tried by just adding it to my CSS file (with an :after-element), but it then hit me I can't target the Wordpress dashboard with the theme's CSS files.

Answer 1

I clearly didn't know how to google, as this page already explains how to do exactly what I wanted. Adapting it a bit, for it to work with different post types, I came up with the following:

add_action('do_meta_boxes', 'replace_featured_image_box');  
function replace_featured_image_box()  
{   
    $post_types = array('bwps', 'page', 'post');
    foreach( $post_types as $post_type ) {
      remove_meta_box( 'postimagediv', $post_type, 'side' );  
      add_meta_box('postimagediv', __('Featured Image <small style="opacity: .6">500*500</small>'), 'post_thumbnail_meta_box', $post_type, 'side', 'low');  
    }
}