How can I set the zoom of the Outlook reading pane?

The Zoom level fixed appears to be fixed now in Outlook 2016. If you click on the zoom number in the lower right corner of your Outlook window, it will pop up a dialog.

Enter the desired zoom level, and click the box to save the preference. Walla! This will cure it for your messages in the reading pane.

This looks to be linked to the DPI settings on your computer as well as the zoom level set in Outlook.

One thing could be to change the text size on the emails -

Customize the font size for Plain Text messages

For Plain Text you can modify the display font via

• Outlook 2007 and previous:
  Tools → Options… → tab Mail Format → button Stationery and Fonts (or just Fonts… in previous versions of Outlook)
• Outlook 2010:
  File → Options → section Mail → button Stationery and Fonts

Source: Enlarge font in the Reading Pane – MSOutlook.info