If you go to app.bookcreator.com and you get the message 'Enlarge browser', it may mean you have to open the browser to full screen. If you do that and you still get the message, it means the screen size of your device is too small to run Book Creator.

Note - you might be able to fix this just be resetting the zoom on your browser to 100% (check under 'View' in the Chrome browser toolbar). It may also help to turn off your bookmarks toolbar, or remove any downloads or other items in the system tray at the bottom of the browser window.
The smallest screen height we support is 570px but that is the viewport of the browser after the url bar and bookmarks bar and windows taskbar etc. have been subtracted.