3D FlipBook - Thumbnails not resizing or following shortcode css or height declarations in category shortcodes

Hello! I have tried 3 different ways of getting the thumbnails to be a different size or slightly different size than an uploaded thumbnail to no avail.

1) I have changed Wordpress thumbnail sizes as per your video, gone back to the PDF book and resaved, and the generated thumbnail on the server does not re-generate.

2) I have tried using a custom thumbnail (which works), but when I tried to reduce the displayed size of that thumbnail with CSS or the height declaration, all that changes is the box around the thumbnail (though the enlarged thumbnail will display, it's just too big and will not reduce based on the shortcode arguments)

Please advise. Thanks!