3D FlipBook - Ref: Wordpress Flipbook - When using ‘Thumbnail and Lightbox’ (wordpress version) how do you adjust the size of the

Ref: Wordpress Flipbook - When using ‘Thumbnail and Lightbox’ (wordpress version) how do you adjust the size of the thumbnail? There is no documentation that indicates how this is done. On you example pages you are showing image covers that are larger than the default setting in the product with no explanation on how you make the image bigger so it can be read / seen properly.