You can change the size of your font when composing email messages in Outlook on the web. To change the size of your font when reading email, use your browser's zoom function.
Change font size when composing email
When composing or replying to an email message, you can change the font or make text larger or smaller.
At the bottom of the compose pane, select Font size.
If you don't see the formatting options menu at the bottom of the compose pane, select Formatting options.
Change font size when reading email
When reading email, you can use your browser's zoom function to make the text on your screen larger or smaller.
In Internet Explorer, select Tools > Zoom.
In Microsoft Edge, select Settings and more > Zoom.