Get all users Deleted Items size:

Get-MailBox |
Get-MailboxStatistics |
select DisplayName,TotalDeletedItemSize

Get a specific users Deleted Items Size:

Get-MailboxStatistics <alias> |
select DisplayName,TotalDeletedItemSize
Purge All Users Deleted Items:
get-mailbox |
Search-mailbox -SearchDumpsterOnly
-DeleteContent
Purge a specific users Deleted Items:
Search-mailbox -identity
<alias>
-SearchDumpsterOnly -DeleteContent 
 
Check Database white space after removal:
Get-MailboxDatabase -Status
| Sort-Object
DatabaseSize -Descending
| Format-Table
Name,
DatabaseSize,
AvailableNewMailboxSpace

Tip! If EMS cannot find search-mailbox cmdlet type this:

Add-PSSnapin Microsoft.Exchange.Management.PowerShell.E2010