Theme Horse

Hi Carol,
Before making any changes in our theme first of all you need to create a child theme to be safe from your code lost. So you need to create a child theme then only customise the code.
Go to theme folder -> functions.php on line no 157 and 158 and you will find this code

	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

Then change the image slider size you want. (e.g:- for slider wide 1440 is width and 500 is height)
You may change this numeric value according to your need.

Thank you!