Add instructions to featured image

WordPress has a hook for that. Here is a sample code:

function featured_image_dimensions( $content, $post_id, $thumbnail_id ){
    $help_text="<p>" . __( 'recommended dimensions - H980px by W450px', 'my_domain' ) . '</p>';
    return $help_text . $content;
}
add_filter( 'admin_post_thumbnail_html', 'featured_image_dimensions', 10, 3 );

Add this code to your functions.php and you should be good to go.