Using classic Outlook for Windows in business environments
As has been suggested, you can change the actual point size of the text you use in your e-mails.
Sometimes, you just want the text to display a bit larger. You can change the zoom for the text in the reading pane. Click the percentage icon at the bottom right, next to the Zoom slider, and adjust the zoom. To save your choice, click the "Remember my preference" option.
See the screen shot below.
Similarly, you can change the zoom in a message which you are composing. Create a new mail and make sure it is open in a separate window. On the Format Text tab, click Zoom. In the Zoom While Composing dialog box, set your preferred zoom.