Put below code in functions.php 

add_filter( ‘admin_post_thumbnail_html’, ‘aviweb_thumbnail_add_description’, 10, 2 );

function aviweb_thumbnail_add_description( $content, $post_id ){
$content .= “<p><b for=\”html\”>Product page (510 * 400)</b></p>”;
return $content;
return $post_id;
}