The following commands are examples to get mailbox sizes for users mailboxes per Exchange database. This gives you both the actual mailbox size and the dumpster (Soft-Deleted) items, items which have been shift-deleted from any folders, or deleted from Outlook’s deleted items folder.
To get a list of all users mailbox sizes per database in descending order, run the following command:
Get-MailboxStatistics –database <DataBase> | Sort-Object TotalItemSize -Descending | ft DisplayName,@{label="TotalItemSize(MB)";expression={$_.TotalItemSize.Value.ToMB()}},@{label="TotalDeletedItemSize(MB)";expression={$_.totaldeleteditemsize.Value.ToMB()}},ItemCount
I refer to this link, it talks about the TotalDeletedItemSize, this refers to the email sitting in the dumpster which is accessible from the users perspective using Outlook’s Recover Deleted Items folder. Once email is purged (Hard-Deleted) from the Recover Deleted Items folder, then this reduces the TotalDeletedItemSize in real time, as long as Single Item Recovery is disabled.
To check if Single Item Recovery is enabled or disabled for a mailbox, run the following command:
Get-Mailbox –Identity <mailbox> | ft name,SingleItemRecoveryEnabled
Marc Kean 
Excellent info! Thanks a million. Due to the recent iOS ActiveSync bug, my database and log sizes were climbing at an unnatural rate with soft-deleted calendar items. This is a quick way to find the troublespots. I really only had to use Get-MailboxStatistics -Database “db” | Select DisplayName, TotalItemSize, TotalDeletedItemSize, but your way is a good way to have Exchange do the search work for me.
Cheers!