Open Outlook
Go to File
Select Options
Click Mail
Click Stationery and Fonts
Under New mail messages, click Font
Choose the font size you want
Click OK
Under Replying or forwarding messages, click Font
Choose the font size you want
Click OK
Click OK again to save changes
To change the reading pane font size, use the zoom slider in the message window
To change the Outlook interface text size, adjust your Windows display scaling settings