// alter the recommended "medium" size in the media uploader

// these are the default sizes
// $defaults['thumbnail'] = __('Thumbnail');
// $defaults['medium'] = __('Medium');
// $defaults['large'] = __('Large');
// $defaults['full'] = __('Full Size');

add_filter( 'image_size_names_choose', 'change_image_defaults' );
function change_image_defaults( $defaults ) {
  if(is_array($defaults) && array_key_exists('medium', $defaults)) {
    // alter the medium size
    $defaults['medium'] = __('medium - recommended for featured Image');
  }
  return $defaults;
}

// add text to the featured image metabox
add_filter( 'admin_post_thumbnail_html', 'change_post_image_metabox' );
function change_post_image_metabox( $content ) {
  return $content.'<p>Recommended size is medium</p>';
}