In Outlook you can free up space by automatically backing up or deleting old items like emails, appointments, tasks, journal enteries and notes. You can back up items for as long as you want. Auto Archive can also delete items if they get to old. These setting are changed by you.

Auto Archive is a simple way to back up or automatically delete items that get to old to keep. It will free up space in your mailbox by moving and compressing your old emails, tasks, notes, appointments, and much more. Default setting are items older than six months old. You can change this to however you want. Auto Archive will ask to be run every week.
To Manually archive a folder hit Go and click Folder List. Select the folder you want archived and hit File >> Archive. Here you can setup the age of the items you want archived. Any item older than the age you put in will be automatically archived.