Is there a plugin out there that can show you how much space is being used per blog on a multisite?
Effectively, it should calculate:
1. the sum (in MB) of each table the DB that has the same 'blog id'
2. the sum of the files that have been added to that site via the Media upload.
3. super bonus: the sum of the files that have been added to any plugins or child themes directories.
In theory, I don't see any rocket science here. So I would be shocked if this doesn't exist anywhere.