The display size of any images, whether that be a flipbook background OR custom embed side widget, is completely dependent on the end users screen resolution. 

If you want complete flipbook coverage. be sure to use a large image (1920X1080 for example), so large screen resolutions will display a full image as well as small screen devices.

You can also choose to repeat (tile) your  image to ensure complete flipbook area coverage on all devices.  Just edit your title and choose the repeating option from the dropdown menu. (initial POSITION and REPEATING options)