WordPress - Filter [ image_size_names_choose ] - Adding Custom Images Sizes to the WordPress Media Library

wp-filter-add_custom_image_sizes

/**
 * Adding Custom Images Sizes to the WordPress Media Library
 *
 * @access public
 * @param array $sizes
 * @return array
 */
function shgist_add_image_sizes($sizes) {
	$addsizes = array(
	    "new-size" => __( "New Size")
	);
	
	$newsizes = array_merge($sizes, $addsizes);
	
	return $newsizes;
}

add_filter('image_size_names_choose', 'shgist_add_image_sizes');