Need some help with Featured Image Slider and Thumbnail Sizing

Hi! I'm having issues with setting image sizes for the featured slider and the automatically generated thumbnail. The Featured Slider is a rectangular image (667x375), and the thumbnail is a square (about 200x200 by the looks of it). I'd like to learn how to do two things: (a) change the thumbnail to be the same aspect ratio as the featured slider (about 2:1) (b) have the ability to use a completely separate image for the thumbnail. As it stands it only looks like the thumbnail pulls from the featured image. (I'm not sure if this is an option in the pro version - if so, let me know and I can upgrade.) So far I have tried resizing the thumbnail in the media settings to 200x100, which didn't work; and I looked through these forums to find any advice - which only led me to this post: https://catchthemes.com/support-forum/topic/change-the-size-of-a-post-thumnail-doesnt-work; which didn't offer a solution for customizing the child theme, only that it was possible. (I can figure out customization with a child theme with some guidance.) If someone could show me the documentation where I could figure out these two items, I'd really appreciate it! Thank you so much! (You can see the featured slider/thumb sizing issue on www.WorkatHomeFAQ.com)