add_action( 'admin_init', 'theme_additional_images' );
 /**
 * Display all image sizes other than the default, thumbnail, medium and large
 *
 */
 function theme_additional_images() {
 global $_wp_additional_image_sizes;
 $get_intermediate_image_sizes = get_intermediate_image_sizes();

echo '<pre>' . print_r($_wp_additional_image_sizes) . '</pre>';
}