Once you start looking at size, you'll notice that the bigger messages are often those that include attachments. If you don't need to store attachments in your mailbox, don't. Instead, as you read your e-mail, use the Save Attachments command on the Microsoft Office Button menu and save the attachments to some convenient location on your hard disk. (You'll need to have a message open to access the Microsoft Office Button menu.)
Once you've saved the attachment, you can delete the message (if you don't need it anymore).
Tip Before you start saving attachments to your hard disk, make sure you have a location in mind that you'll remember later. For example, if the attachment is a photo, you may want to save it in the Pictures or My Pictures folder. Also, when you save the attachment, make sure you give it a name that will help you remember its contents and context.