How to increase the font size for the EPUB output.
Pressbooks produces reflowable ebooks. The user controls the font size.
Normal ebooks that you’re reading (such as a regular Kindle book or novel, etc.) are what’s called “reflowable.” With a reflowable ebook, on the page you can resize the fonts and the book just reorganizes itself. You can make bigger fonts, smaller fonts and the page will just flow to the next thing. You might be reading landscape; you might be reading portrait and the text and images will fill up the page as you’ve specified them to fill up in the ebook.
Source: https://pressbooks.com/blog/10-things-to-know-about-designing-an-ebook/
If you want to change something like this, use custom CSS:
https://guide.pressbooks.com/chapter/customizing-your-exports-with-custom-styles/