There are two ways to temporarily increase or decrease the zoom level in the Outlook Reading Pane.
Use the Zoom control at the right, bottom of the Outlook window.
If you are using a mouse with a wheel control, click the Reading Pane, press Ctrl, and roll the scroll wheel. Rolling the wheel away from you makes the contents larger, rolling the wheel toward you makes the contents smaller.
The zoom affects only the current item. When you select another item or folder, the Reading Pane returns to the default 100% size.
Note: You can’t change the text size for the Reading Pane header.