Viewing Current Mailbox Size, Message Count, and Last Logon - Exchange server

1. Open Exchange Management shell
2. Get-MailboxStatistics [-Server 'Server' | -Identity 'Identity' | -Database 'Database']

Here are some examples of using this cmdlet:
Get-MailboxStatistics -Server 'corpsvr127'
Get-MailboxStatistics -Database 'Corpsvr127\First Storage Group\Engineering'
Get-MailboxStatistics -Identity 'cpandl\williams'