我最终做到了：

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 );
}