New size will show as media option
see: https://pippinsplugins.com/add-custom-image-sizes-to-media-uploader/

function pw_add_image_sizes() {

    add_image_size( ‘example-thumb’, 300, 300, true );

}

add_action( ‘init’, ‘pw_add_image_sizes’ );

function pw_show_image_sizes($sizes) {

    $sizes[‘example-thumb’] = __( ‘Example Thumb’, ‘pippin’ );

    return $sizes;

}

add_filter(‘image_size_names_choose’, ‘pw_show_image_sizes’);

final line adds the size option in the media manager