To view how many items are in each mailbox on a database and to see if people have hit their mailbox limit use the cmdlet below. Change the red text to the name of your database name.
Get-MailboxStatistics -database "Mailbox Database"
To view an individuals quota use the cmdlet below. Remember to change the bits in Red to match your organisation.
Get-Mailbox "Joe Bloggs" -Organization orgname | Format-List *Quota