Have you ever need a list of yours mailbox and the size of it. You can find this in system manager, but you cannot export this list.
With Powershell you can get a list that you example can import in excel.
First you need to download and install powershell, you can download powershell from here:
http://www.microsoft.com/windowsserver2003/technologies/management/powershell/default.mspx

After you have downloaded and install it, type this command in the powershell

get-wmiobject -class Exchange_Mailbox -Namespace ROOT\MicrosoftExchangev2 -ComputerName servername | select-object MailboxDisplayName,TotalItems,Size

Change “servername” to you servers name example exchange01