The fields of "Plan my book with me" are small and I have to scroll a lot. Can I make it bigger?

Yes, from version 1.9.21. The fields now grow with their text, so everything you or BookyAI wrote is visible without scrolling inside a field; you can still drag the corner of a field to make it taller. The window itself can also take the whole app window: press the arrows button at the top right, next to the cross ("Bigger window"). Press it again to go back to the usual size. BookyAI remembers your choice for next time.