HOME PAGE - Reducing slideshow image sizes
Kagami is a full-screen theme. This means that images and content expand from edge to edge of the screen.
It can become problematic on slideshow images which can quickly grow big, as Kagami do not resize your images to make sure that all the content appear as you have designed it.
To make the image smaller, you will therefore need to modify its aspect ratio. In your case, you will need to make the images height smaller, but keeping the same width. For instance, if your slideshow images are 1000x900, keep the same width, but reduce the height. You could therefore try with a size such as 1000x800, and reduce again if necessary, until you achieve your desired size.