Here is a quick way to find the largest mailboxes in Exchange 2010 in the Exchange Management Shell (EMS) using the “Get-MailboxStatistics Cmdlet:
Get-MailboxStatistics -server SERVERNAME |sort-object -Property totalitemsize -des |select-object Displayname, ItemCount, totalItemSize -first 10
Replace SERVERNAME with your server, and you are in business. You can get creative and change “10” to what ever number you want. You can also add ” > Top10.txt” to the end of this command and have the information print out to a text file that you can attach to an email.