Hi julie.pett,
if you like to change the slider image size then go to functions.php and on line no 152 and 153 you will find this below 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
change you slide image size according to your needs. But create a child theme before making any changes on it. After changing the slider size install regenerate Thumbnail plugins and regenerate the images. If your slider doesnot work then first make a direct change in the 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
to
add_image_size( 'slider-narrow', 2076, 1000, true ); // used on Featured Slider on Homepage Header for narrow layout
add_image_size( 'slider-wide', 2880, 1000, true ); // used on Featured Slider on Homepage Header for wide layout
and then regenerate thumbnails.
Thank you!