Presumably you re using a web browser to access your email on your computer. If you are it is your web browser that sets the font size.
If you are using Microsoft Edge you need to select the Settings button in the top-right of the Microsoft Edge web browser. Select Settings from the menu.
The Microsoft Edge settings page will now appear. Type “Fonts” into the search box. From the search results, select Customise fonts. On the Customise fonts screen you can choose which fonts size Microsoft Edge will use.
Select the size that suits your needs.
If you are using a different browser there will be some similar way of changing it.
You can also change the page size by zooming in or out by using the keyboard Control and plus + button to zoom in and the Control and minus - button to zoom out.
If you are using an email client/app you need to check the settings for that app and there will be a way of changing it.
