You can easily adjust the font size in your theme settings under Typography.
The default settings follow industry standards, but feel free to customize it to your liking for your book. Keep in mind that the Font Size you choose will be applied universally throughout your entire book.
You can make changes to your theme on the Formatting Tab by clicking the three dots next to the theme name and selecting Edit or Edit as New Theme.
For more details on how to set up or adjust your theme settings, you can refer to this tutorial: https://www.atticus.io/create-a-custom-theme/ It provides detailed instructions and tips on creating and modifying your theme.