Open Outlook
Go to File
Select Options
Click Mail
Under Compose messages, click Stationery and Fonts
Choose the font size for new mail messages
Choose the font size for replying or forwarding messages
Click OK
To change reading pane font size, use the zoom slider or press Ctrl and + to zoom in
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 fonts
Click OK to save changes