In Outlook 2010, Outlook 2013, and Outlook 2016 this dialog is at File, Options, Mail.
It applies to both messages sent as plain text and HTML messages that you choose to read as plain text. This font is used both in open messages and in the reading pane. The bottom setting on this dialog is the font used for plain text messages and replies. The middle two options, for New mail messages and Replies and Forwards, controls the default fonts used for HTML and RTF messages and apply only to messages you send. In Outlook 2007 and newer, you can change the fonts used for reading plain text messages from the Stationery and Fonts dialog in Options. Plain Text | HTML | Rich Text | WordMail | More Information Outlook 2010 and newer
See Adjusting Outlook's Zoom setting for more information. Instead of changing the font in messages, you can adjust the Zoom.
