I assume you are talking about just the file-uploads and downloads, and not the webserver html file downloads.
In your upload routine there has to be a copy command, you can add a filesize checker there and add the amount to the upload stats.
As for download, you'd need top build a wrapper script that will offer the file to the client, and add the filesize to the total while doing so.
You'll have to think of a safe way of storing the totals though.
Simply writing it to a file is not multi-user safe.