This Power Shell cmdlet will show all of the mailbox sizes and their quotas.  It will also let you know if the mailbox has exceeded it’s quota and disabled as a result.  It outputs the username, the total item size (sorted highest to lowest), and their storage limit status.

Get-MailboxStatistics -Server SERVERNAME | Sort-Object -Property TotalItemSize -Descending | Format-Table DisplayName,TotalItemSize, StorageLimitStatus > FILENAME.txt
Get-mailbox -Identity UID

Replace SERVERNAME and FILENAME with your server name and the name and path of a text file that you want these statistics output to.