Best size for image in feature slider?

Hi! I really like the Customzr theme. I want to create a collage image that will fit exactly inside the feature slider on the front page. I also want the image to scale properly on all devices. Like on this website: https://www.donovancreed.com/ (See their feature slider showing the author’s books, please.)

How do I do this in this theme? What is the best size in pixels for an image like this? That is all I need to do. Otherwise the theme is working perfectly!

Thanks for a great FREE theme!