Set custom banner image size with wp_customize

Turns out it’s fairly straightforward, just a case of knowing what to search for

https://codex.wordpress.org/Class_Reference/WP_Customize_Manager

In my tyc_banner_image function I added the last four arguments below.

$wp_customize->add_control( new WP_Customize_Cropped_Image_Control($wp_customize, 'tyc_banner_image-control', array(
            'label' => 'Image',
            'section' => 'tyc_banner_image-section',
            'settings' => 'tyc_banner_image-image',
            'flex_width'  => true,
            'flex_height' => true,
            'width'       => 1920,
            'height'      => 500,
        )));