Can I change the font or the font size?

Yes, you can change the font and the font size right from the Reader Menu. Lightly tap the screen when you're reading a book to bring up the menu, click on the Aa option, then make changes as you like. These changes will be persistent across all books you're reading in Fable until you change them again. You can also use this menu to change the reading orientation or switch from Day to Night Mode.