You can see the Single Instance Store ratio by graphing / viewing the "Single Instance Ratio" counter on the MSExchangeIS Mailbox object (and the instance of that object corresponding to your mailbox store). It won't show you the size in bytes, though-- it just shows you the average number of reference for each message in the store.
Based on your comments on Massimo's answer, it sounds like you're running Exchange Standard Edition. You're not going to be able to create another store.
You could consider bringing up a temporary install of Exchange onto another machine, moving the mailboxes there, removing the mailbox store database on the "source" Exchange Server computer (so that Exchange starts there with a blank database) and moving the mailboxes back. Your users won't see downtime (though I don't recommend moving someone's mailbox while they have Outlook open), and depending on the amount of data and the speed of the servers and network involved you could probably get that done fairly quickly (perhaps over an evening).
Given that I'm at a loss to describe why your database is so large, though, I can't really say that this would help. It's worth a shot, I suppose. (I'd be tempted to go to Microsoft Product Support Services at this point, though. I'm not one to jump on paid tech-support very quickly, but the level of hassle associated with the mailbox move might just be higher than the $300.00 to talk to Microsoft...)