Featured Image Size in Metabox

plugin.php

<?php
/**
 * Plugin Name: Featured Image Size in Metabox
 * Description: Adds 123x456 to the metabox title in featured image.
 */

add_action( 'do_meta_boxes', 'my_do_meta_boxes' );
function my_do_meta_boxes( $post_type ) {
	global $wp_meta_boxes;
	if ( ! current_theme_supports( 'post-thumbnails', $post_type ) || ! post_type_supports( $post_type, 'thumbnail' ) )
		return;

	print_r($wp_meta_boxes);

	foreach ( $wp_meta_boxes[ $post_type ] as $context => $priorities )
		foreach ( $priorities as $priority => $meta_boxes )
			foreach ( $meta_boxes as $id => $box )
				if ( 'postimagediv' == $id )
					$wp_meta_boxes[ $post_type ][ $context ][ $priority ][ $id ]['title'] .= ' 123x456';

	remove_action( 'do_meta_boxes', 'my_do_meta_boxes' );
}

very helpful!