这段代码可以工作，尽管我怀疑它可能更高效，或者被更好的过滤器替代。我根据这个问题的答案修改了这段代码，这是我花了几个小时试用的所有googles中最好的选择。enter link description here

您需要将“full”更改为您想要的大小（在两个位置）。您还可以根据需要更改其他数组元素（在“提取”函数之后）。

我愿意用更简单/更有效的方法来做这件事。

 function change_image_size ($output, $attr) {
        global $post;  // needed to use in the id element
        extract(shortcode_atts(array(
            \'order\' => \'ASC\',
            \'orderby\' => \'menu_order ID\',
            \'id\' => $post->ID,
            \'itemtag\' => \'dl\',
            \'icontag\' => \'dt\',
            \'captiontag\' => \'dd\',
            \'columns\' => 3,
            \'size\' => \'thumbnail\',
            \'include\' => \'\',
            \'exclude\' => \'\'
        ), $attr));
        // here\'s where you can change/add an attribute to the shortcode
        $attr[\'size\'] = \'full\'; // change \'full\' to desired size
        $id = intval($id);
        if (\'RAND\' == $order) {
            $orderby = \'none\';
        }

        if (!empty($include)) {
            $include = preg_replace(\'/[^0-9,]+/\', \'\', $include);
            $_attachments = get_posts(array(\'include\' => $include, 
\'post_status\' => \'inherit\', \'post_type\' => \'attachment\',
 \'post_mime_type\' => \'image\', \'order\' => $order,
 \'orderby\' => $orderby));

            $attachments = array();
            foreach ($_attachments as $key => $val) {
                $attachments[$val->ID] = $_attachments[$key];
            }
        } elseif (!empty($exclude)) {
            $exclude = preg_replace(\'/[^0-9,]+/\', \'\', $exclude);
            $attachments = get_children(array(\'post_parent\' => $id, \'exclude\' => $exclude, \'post_status\' => \'inherit\', \'post_type\' => \'attachment\', \'post_mime_type\' => \'image\', \'order\' => $order, \'orderby\' => $orderby));
        } else {
            $attachments = get_children(array(\'post_parent\' => $id,
 \'post_status\' => \'inherit\', \'post_type\' => \'attachment\', 
\'post_mime_type\' => \'image\', \'order\' => $order,
 \'orderby\' => $orderby));
        }

        if (empty($attachments)) {
            return \'\';
        }

    // Essentially these are only changes I\'ve made
    // you can change the $output to your needs; including changing \'full\' to your desired image size.
        $output = \'\';
        foreach ($attachments as $att_id => $attachment) {
            $output .= \'<figure>\' . wp_get_attachment_image($att_id, \'full\') .

\'<figcaption>\' . wptexturize($attachment->post_excerpt) . 
\'</figcaption></figure>\';
        }   // change \'full\' to desired size

        return $output;
    }
    add_filter(\'post_gallery\', \'change_image_size\', 10, 2);