It looks like you're using Internet Explorer 11 or older. This website works best with modern browsers such as the latest versions of Chrome, Firefox, Safari, and Edge. If you continue with this browser, you may see unexpected results.
To change the overall Font size in Outlook, change your Windows settings. This will change all the settings for all Windows applications (not just Outlook) at once so you don’t have to change it for each individual application.
Right click on an empty spot on your Desktop and choose “Screen Resolution”.
Click “Make text and other items larger or smaller”.
You can now directly choose to make the fonts larger. If you prefer a different font size, you can click on the “Set Custom text size (DPI)” on the left and type any percentage you like. Click Apply.