Open Outlook
Go to File
Select Options
Click Mail
Under Compose messages, click Stationery and Fonts
Choose the font size you want for new emails
Click OK
To make text bigger while reading an email, use the zoom slider at the bottom right
To make the reading pane text larger, open the email and increase zoom
To change the font size in the Outlook interface, go to View and adjust the reading pane or display settings if available
On Windows, you can also use system display scaling to make Outlook text appear larger