Originally Posted by SCION
You can set it permanently. Open a book, tap to bring up the bottom menu, click on "Theme" icon, then select "Edit". That's where you set your reading options. You've been missing out on much if you haven't discovered this!
Edit: The above info doesn't apply to font size. Sorry. Click the "Display" icon instead to change font size.
I already have a couple of custom themes but as you mentioned the font size is set elsewhere. What I don't like, is that a casual slide on the screen may change the size of the font. Annoying. It would be good to have that as an option rather as a "feature." I usually set the font size once and then forget it.