1) Open 'Exchange Management Shell'

2) Type 'Get-MailboxStatistics -Database "Mailbox Database" | Select DisplayName, ItemCount, TotalItemSize | Export-CSV c:\mailboxinfo.csv' (without the quotes)

3) Open that file in Excel.

You could also use any of the below variables in the above command and just seperate them with a comma:
  • AssociatedItemCount
  • DatabaseName
  • DeletedItemCount
  • DisconnectDate
  • DisplayName
  • Identity
  • ItemCount
  • LastLoggedOnUserAccount
  • LastLogoffTime
  • LastLogonTime
  • LegacyDN
  • ServerName
  • StorageGroupName
  • StorageLimitStatus
  • TotalDeletedItemSize
  • TotalItemSize
http://www.wallaceit.co.uk/microsoft-exchange-server-2007/view-export-exchange-2007-mailbox-sizes/166/article.aspx