You would use Mailbox Cleanup tools. In Outlook 2007, click Mailbox Cleanup on the Tools menu, and click the 'View Mailbox Size'. Unfortunately, the data totals are listed in kilobytes (KB), so it may seem misleading. Remember that roughly 1,000 KB is 1 MB (megabyte). You are allowed 500 MB in your mailbox, so that's equal to 500,000 KB. For example, a mailbox that is 39,500 KB in size translates to 39.5 MB.