Olivia here from the Shopify Team. Thanks for sharing your question with us.
As I was looking over your question, I wondered if you might find this tutorial helpful written by my colleague on editing hero images. You mentioned you wanted to "reduce the size of the image" in your slideshow so I trust this will be the perfect guide for you.
If however, you are looking to make other design changes to the layout of your theme please share as much context and/or examples with me of your intended outcome as you can so I can get you a solution.