Last updated: November1, 2017
Change the text size of the sidebar, message list, and reading pane (Insider Fast only)
Insiders: You now have the ability to increase the display size of the sidebar, mail list, and reading pane text. This feature is available in version 16.7 (171017) and later.
Go to Outlook > Preferences > Fonts.
Move the slider left or right.
This feature will be available to all users at a later date.
Change the font size for received messages
To increase or decrease the font size of messages you have received:
Select a message from your Inbox or other folder, including Sent and Drafts.
Select Format > Increase Font Size or Decrease Font Size.
Note: Incoming HTML messages often have font styles and font sizes applied to the text. The default font and font sizes that you select will not affect the formatting of these incoming messages.
Choose a default font for composing messages
Select Outlook > Preferences > Fonts.
Select the options you want.