Hi,
Please go to the plugin settings page and find "Gallery image size", where you can select height and width for thumbnails.
If you want the image's size to change depending on the screen's dimensions - unfortunately, this functionality is not available, but if you want, you can order a paid customization and provide details in a private ticket on our support forum http://support.bestwebsoft.com/.
Sincerely,
BestWebSoft Support Team
