Hi Casey,
Upload the smaller size of image you want to take less space in slider. All image size must be same. If you like to do it from coding then inside functions.php on line no 155 to 160 there we have defined the size of the image.
add_image_size( 'slider-narrow', 1038, 500, true ); // used on Featured Slider on Homepage Header for narrow layout
add_image_size( 'slider-wide', 1440, 500, true ); // used on Featured Slider on Homepage Header for wide layout
You may change this size as you want. After changing the size of image then reupload the image again or use regenerate thubmails plugins to regenerate your image. First of all before making any changes create a child theme then only customize the code to be safe while uploading to our new version.
Thank you!