I just wanted to add that the DA File Manager has a column for "size" but for directories it's misleading as it's taken directly from the Unix/Linux "ls" command, and it's simply the size of the file that makes up the directory listing; it's not the aggregate size of the directory contents.
As far as I know, it's impossible to measure the usage caused by a specific directory, however it is possible to measure traffic on a server per ip allocated to the server.
I experimented with this once, but I can't find the files, nor the report about it... (which usually means it was going to be a pain in the *rse too much)
You can try creating a per ip counter in iptables, however you shouldn't as me, as I don't know the correct command at the time.