Make the book itself resizeable, make full screen setting toggle to auto resize the book to maximum.

It can be resized on Customize Desktop.

The book itself is resizeable, - by holding down the 'Ctrl' key and using the scroll wheel on the mouse.

Thanks for finding that :)
But, the problems remain:

-That it is so difficult to know.
-That the control handles on customize desktop are tiny tiny tiny! Again, many readers have visual issues, this is no good to them if they cannot see the nodes to resize it!
(Also this method causes error and crash. I sent the bug report. But on restart of program the new size was kept at least.)
-The control key plus mouse wheel method is at least workable for those whose eyes are not great, but it would be difficult for older hands to control. So there should still have another method to resize book, on desktop a plus sign to click maybe? Or on right click menu for book change "zoom" option to say "zoom only text" and add there also the option to resize the book entirely?