Is there a way to tell who is using the mail space and how much they are using?
Currently, you can only view mail space usage on a per user account basis. To do this, you will need to login to SquirrelMail (via http://yourdomain.com/webmail). Go to "Options", then "Folder Preferences" and look at the bottom, this will allow you to configure your prefences to allow you to get the size and message totals for your folders.
If you are using IMAP, you can also configure your email client (such as Outlook) to allow you to view your usage as well. You should view your mail client documention on how to get this setup.