Exchange 2007 had a nice screen that showed, for each mailbox, the number of items and the size of the mailbox. The closest that I’ve been able to find for Exchange 2007 is this command to be run from the Management Shell.

Get-MailboxStatistics

Microsoft have a technote about the command here.

I also found a useful command line here which is shown below:

Get-MailboxStatistics where {$_.TotalItemSize -gt 100MB} sort $_.TotalItemSize FT DisplayName,ItemCount,TotalItemSize

Basically the list shows the Display Name, Item Count and Mailbox Size for Mailboxes greater than 100MB.