exchange server - Locating largest mailboxes

Does anyone know of any good Exchange 2003 Reporting. I would Like to See the Biggest Mailboxes and who is sending out the biggest emails.

To view user mailbox size.

Open "System Manager"
then click "server"
then click "storage Group"
then click "Mailbox Store"
then click "Mailboxes"

I'm not sure about who's sending larger e-mails. For me having a quota size on send e-mails is enough. This article should help in this regard.

MsExchange.org