pre\\u get\\u posts成功了（感谢Pieter和cybmeta！

以下是解决方案：

function hwl_home_pagesize( $query ) {
if ( is_post_type_archive( \'motivation\' ) ) {
    $query->set( \'posts_per_page\', 9 );
    return;
}
if ( is_post_type_archive( \'blog\' ) ) {
    $query->set( \'posts_per_page\', 14 );
    return;
}
}
add_action( \'pre_get_posts\', \'hwl_home_pagesize\', 1 );