$f = './path/directory';
$io = popen ( '/usr/bin/du -sk ' . $f, 'r' );
$size = fgets ( $io, 4096);
$size = substr ( $size, 0, strpos ( $size, "\t" ) );
pclose ( $io );
echo 'Directory: ' . $f . ' => Size: ' . $size;
?>

kaynak : http://stackoverflow.com/questions/478121/php-get-directory-size