Custom Image Size in Insert Media

I ended up doing this:

add_filter( 'image_size_names_choose', 'jss_custom_image_sizes' );
function jss_custom_image_sizes( $sizes ){
    $custom_sizes = array(
        'customsize1'        =>        'Post Rectangle Image',
        'customsize2'        =>        'Post Square Image',
    );
    return array_merge( $sizes, $custom_sizes );
}