Get the size of the attached file
filesize( get_attached_file( $attachment->ID ) );
function getSize($file){
$bytes = filesize($file);
$s = array('b', 'Kb', 'Mb', 'Gb');
$e = floor(log($bytes)/log(1024));
return sprintf('%.2f '.$s[$e], ($bytes/pow(1024, floor($e))));
}