In Exchange 2010 you can see how big your mailbox databases are, and how much white space they have, by running Exchange Management Shell command.
AvailableNewMailboxSpace, also called database white space, is database pages that were previously used to store items and mailboxes that have been deleted and are available for reuse by Exchange to store new mailboxes, items and attachments. This space will be reused as soon as new content will be written to the database and before the database file size grows.
Run the following command in Exchange Management Shell:
Get-MailboxDatabase -Status | Sort-Object DatabaseSize -Descending | Format-Table Name, DatabaseSize, AvailableNewMailboxSpace