Open Outlook
Go to File
Select Options
Click Mail
Under Compose messages, click Stationery and Fonts
Choose the font for new mail messages
Choose the font for replying or forwarding messages
Choose the font for plain text messages if needed
Click OK
Click OK again to save changes
To change reading pane font size, use the zoom slider or press Ctrl and scroll the mouse wheel
To change the message list font size, go to View
Select View Settings
Choose Other Settings
Adjust the font size for column headers and row font
Click OK to apply changes