One way is to create a File System filter to collect all the files from the folder in question, then write a SQL report that will do SUM(file_size) GROUP BY path (pseudocode, as I don't have the column names handy).
Another way would be to create a WCI filter using Powershell as in this article (http://blogs.technet.com/b/heyscriptingguy/archive/2012/05/25/getting-directory-sizes-in-powershell.aspx) to gather the directory name and size information for the element normal XML.
The second approach takes less space in the VCM database, which may be better if you don't need the file details.