Demo using WP's admin_post_thumbnail_html filter to insert content in featured image metabox

wp_admin_post_thumbnail_html_example.php

<?php
/**
 * Adds a detailed UI to WP's thumbnail HTML markup used in the Feature Image metabox.
 *
 * Adds a description of featured images, a listing of the formats used by GV and
 * previews of how the images will appear in features slider and thumbnails.
 * 
 * Note that $thumbnail_id is only saved when the post is saved. Immediately after the featured image is changed
 * this value is updated, but not the postmeta field in the database (nor OUR fields which are tied to it) 
 * SO: 
 * - empty $thumbnail_id means EITHER the post didn't have a thumbnail when it was saved OR that the "remove featured image" button was just pressed.
 * - non-empty $thumbnail_id means EITHER the post was saved with a featured image OR an unsaved featured image was added and this is the preview.
 * 
 * @param string $output The thumbnail HTML markup
 * @param int    $post_id The post ID of the post being edited
 * @param int    $post_id The post ID for the attachment post
 */
function gv_filter_admin_post_thumbnail_html($output, $post_id, $thumbnail_id = ""){

	// ...

	/**
	 * If this post has a featured image set
	 * Note: Could be current (saved in DB) value, or temporary (used for generating preview) thumbnail_id
	 */
	if ($thumbnail_id) :

		/**
		 * Get the post id of the attachment that was chosen as featured image
		 */
		$featured_image_post_id = $thumbnail_id;

		/**
		 * Get the data arrays ['url', 'width', 'height', 'is_intermediate' ] about our CUSTOM sizes
		 * Note: WP will return the largest size available even if it's too small, so these dimensions
		 * are likely to describe "featured_image_small" (CUSTOM) or "full size" (original upload).
		 */
		$featured_image_large_data = wp_get_attachment_image_src($featured_image_post_id, 'featured_image_large');
-		
-		// For the record: No stated size string gives you the old small square 'thumbnail' size
-		//$featured_image_thumbnail_data = wp_get_attachment_image_src($featured_image_post_id);
		
		/**
		 * Determine the width and height of the file chosen for featured_image_large
		 */
		$featured_image_large_width = $featured_image_large_data[1];
		$featured_image_large_height = $featured_image_large_data[2];

		// ...
	endif;

	return $output;
}
add_filter('admin_post_thumbnail_html', 'gv_filter_admin_post_thumbnail_html', 10, 3);

?>