Thank you for your question!
You can adjust the font sizes of elements on your website by using the Fonts option in your Customizer. Changing the size value using the Main Text option will increase the size of the font in your footer. However, this will also scale many of the other fonts used on your pages and there isn’t a way to isolate your changes to only the footer.
If you ever have any other questions in the future please do not hesitate to ask, we are always happy to help!