You can use the Exchange Management Console to view who last logged on to a mailbox, last logon date and time, mailbox size, and message count by completing these steps:
1. In Exchange Management Console, expand the Recipient Configuration node and then select the related Mailbox node.
2. Double-click the mailbox with which you want to work.
3. On the General tab, the Last Logged On By text box shows who last logged on to the mailbox and the last logon date and time.
4. On the General tab, the Total Items and Size (KB) areas show the number of messages in the mailbox and the current mailbox size in kilobytes, respectively.

If you want to view this same information for all mailboxes on a server, the easiest way is to use the Get-MailboxStatistics cmdlet. The syntax for this cmdlet looks like this:
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'