Change the font or font size of the message list
The message list appears next to the Navigation Pane in Outlook. The default font setting is Segoe UI regular 8-point.
You can customize the font and its size. For example, you can change message list to use a different font or a smaller or larger font size.
In Mail, on the View tab, in the Current View group, click View Settings.
Tip To open the Advanced View Settings dialog box, right-click the message list heading where Arrange By appears, and then click View Settings.
Click Other Settings.
Under Column Headings and Rows, click Row Font.
In the Font dialog box, click the font, font style, and size that you want to use.
On the Font, Other Settings, and Advanced View Settings dialog boxes, click OK.