Shopify slideshow image size depends on the slider used in the theme.

The recommended Shopify slideshow image size is 1200px to 2000px width, and 400px to 600px height. When switching to a mobile or small screen view there is some resizing and cropping that is done, so the image won’t be identical. We would recommend, when setting up the images for the slide, to make them middle aligned. This should make things look a bit better, as it will be the top or bottom and left or right that will get cropped out instead of part of the middle.

If you are still confused about the slideshow image size contact us we will help you.