Well, on Unix you would use the command 'du' to to do that. So you could run that with one of the program execution files. How about something like:
$dir = "/path/to/dir";
chdir($dir);
$output = du -sh;
echo $output;
(Those are backticks by the way, see the Execution Operators section of the manual for more, and make sure you check your input before using them for anything else.)
If you're on Windows, well, I don't know. And I don't care either. 🙂
adam