Some users prefer the better visibility of a larger font size for reading their messages in the Reading or "preview" pane. You can make the text or font in the pane larger or smaller by using the scroll wheel on your mouse. Using the scroll wheel is the only way you can change the text size, although more options are being considered for a future version of Outlook.
Click in the Reading Pane, press CTRL, and roll the scroll wheel. Rolling the wheel away from you makes the text bigger, rolling it towards you makes the text smaller.
Note: The text size doesn't persist when you change views. If you go to your Calendar, for example, or another mail folder like Sent Items, the text goes back to the default size.