While this is now something you can do in one line in Exchange 2007 and 2010 if your migrating or managing 2000/3 you may want to get the Size and Item Count details of one particular mailbox folder for all users on a server eg how big the Inbox,SentItems or Calendar folder is. In Exchange 2003 you could get the Mailbox Size via WMI but to get an individual folder size you need to access the Mailbox itself (eg you can use pfdavadmin etc). To do this via Power shell for all users on the server you first need an ADSI query to get all the mailboxes on a particular server then you can use with WebDAV via the virtual admin root or MAPI via RDO\Redemption http://www.dimastr.com/redemption/ which works well in Powershell. I have two scripts that use show how do to this using each of these API's which I've posted a downloadable copy here To run these scripts you need to use the Netbios name of the server as a commandline parameter the RDO version of the script looks like $snServer...
Pushing the Envelope in Messaging and Office 365 Development