There are two options to adjust the size of the book in the editor.

In the View tab you can choose one of the two options: Zoom to Fit or 100%.  

To fit the broadside to the size of the current window, click the Zoom to Fit button on the toolbar.

100% will zoom the book to 100% of the default size.

The current zoom level will be shown in the right bottom corner of the editor. You can enlarge or reduce a page to any size allowed – within the range of the minimum and maximum zoom levels. By dragging the scroll box right and left you can set the custom zoom level from 50% up to 400%.