If you are referring to the slider that is displayed on the front page then its height can be adjusted according to the image size. If you want slider of smaller height then you can upload the image of smaller height as slider will adjust its height according to the image height.
If you are referring to the image that is displayed on the header of other images then you can decrease the size by adding below given custom CSS.
For this go to Admin Panel > Appearance > Customize > Additional CSS and paste the CSS.
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