您可以使用：

<?php $image_attributes = wp_get_attachment_image_src( $attachment_id, array(1024,1024) ); ?>
<a href="<?php echo $image_attributes[0]; ?>">xxx</a>

如果图片太小，将显示原始图片，否则将显示裁剪版本（1024x1024）。

更新：没有正确理解您的问题。以下是您需要的：

首先，为自定义的最大大小添加缩略图大小（true是硬裁剪，false是软比例裁剪）

add_image_size( \'custom_size\', 1024, 1024, true );

function custom_image_media_send_to_editor($html, $attachment_id, $attachment) {
    $attachment_ = wp_get_attachment_image_src( $attachment_id, \'custom_size\' );
    $attachment[\'url\'] = $attachment_[0];

    $post =& get_post($attachment_id);
    if ( substr($post->post_mime_type, 0, 5) == \'image\' ) {
        $url = $attachment[\'url\'];
        $align = !empty($attachment[\'align\']) ? $attachment[\'align\'] : \'none\';
        $size = !empty($attachment[\'image-size\']) ? $attachment[\'image-size\'] : \'medium\';
        $alt = !empty($attachment[\'image_alt\']) ? $attachment[\'image_alt\'] : \'\';
        $rel = ( $url == get_attachment_link($attachment_id) );

        return get_image_send_to_editor($attachment_id, $attachment[\'post_excerpt\'], $attachment[\'post_title\'], $align, $url, $rel, $size, $alt);
    }

    return $html;
}
add_filter(\'media_send_to_editor\', \'custom_image_media_send_to_editor\', 11, 3);