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, open the email
Press Ctrl and + to zoom in
Or use the Zoom control at the bottom right
To change the Outlook interface font size, adjust your Windows display scaling settings
On Windows, go to Settings
Select System
Click Display
Increase Scale and layout
Restart Outlook if needed