What version of Outlook are you running?
Using Outlook 98, right click on 'Outlook Today' and select 'Properties for 'Mailbox ...'' from the shortcut menu.
When the properties dialog box displays, click on the Folder Size... button. The size of your mailbox including all subfolders etc will be calculated and displayed.
You cannot do this in Outlook 97, don't know about Outlook 2000.
Does this help?