Hi There,
You didn't say which version of Outlook you're using so I'm giving you a
general idea of how this is done. Steps may vary from what I describe.
At your Inbox, click on the View button on the toolbar. Scroll to Current
View. From the list that comes up, choose Customize Current View. Click on
the Other Settings button and change your fonts and font sizes there. This
should hopefully accomplish what you're looking for. If not, post back.