Exchange 2007 Listing of Mailboxes
Exchange 2007 does not have the same mailbox size view that Exchange 2003/2000 has. To create a listing of mailboxes and their respective sizes, run the following command in the Exchange Management Shell:

get-mailboxstatistics -verbose | sort-object totalitemsize -descending | format-table totalitemsize, displayname >c:\output.txt

The results will be created in a file at c:\output.txt.

More on this from Brad Dinerman, who has lots of excellent tips.