I don't think you are going to get that function, besides why worry about folder size, just assign a data size level to a user and count the size of the files, they don't even have to be in the file system or they could be spread across multiple folders. By working on folder size you are painting yourself into a corner
+1
You have a database record for each document stored in either the filesystem or database. Having a 'filesize' attribute for each document and maintaining a running total is easy. This can also be verified by a weekly, monthly scheduled process that can verify it and sending off any alerts to your users / admins etc.
If you have a large amount of files don't to the full size calculation every time a file is added, only do the addition of the new value to the running total.