Hey there Charlotte,
This is Alex from the Shopify Guru team, happy to assist you today!
The height of the slideshow can be adjusted by changing the image sizes of the slideshow images.
To make the slideshow smaller, you can use landscape images, which are images that are wider than they are tall.
Although the theme editor suggests that you upload images that are 1000px wide by 500px high, you can use smaller images. A width of 1000px is recommended so that your slides don't have any white space on the left and right on larger displays. Images that have a smaller width will be centered in your slideshow.
You can read more about this in Image Tips for Supply.