Changing size of linked image in lightbox

Hi
Hoping someone can help.

Following other posts I have managed to set my featured images to open the larger version of themselves in a slimbox – however, how can I change the size of the main image that’s in the overlay?

This is the code that has been edited in multi-post-thumbnails.php – from line 269…

* Retrieve Post Thumbnail.
		 *
		 * @param string $post_type The post type.
		 * @param string $thumb_id The id used to register the thumbnail.
		 * @param int $post_id Optional. Post ID.
		 * @param string $size Optional. Image size.  Defaults to 'thumbnail'.
		 * @param bool $link_to_original Optional. Wrap link to original image around thumbnail?
		 * @param string|array $attr Optional. Query string or array of attributes.
		  */
		public static function get_the_post_thumbnail($post_type, $thumb_id, $post_id = NULL, $size = 'post-thumbnail', $attr = '' , $link_to_original = false) {
			global $id;
			$post_id = (NULL === $post_id) ? get_the_ID() : $post_id;
			$post_thumbnail_id = self::get_post_thumbnail_id($post_type, $thumb_id, $post_id);
			$size = apply_filters("{$post_type}_{$post_id}_thumbnail_size", $size);
			if ($post_thumbnail_id) {
				do_action("begin_fetch_multi_{$post_type}_thumbnail_html", $post_id, $post_thumbnail_id, $size); // for "Just In Time" filtering of all of wp_get_attachment_image()'s filters
				$html = wp_get_attachment_image( $post_thumbnail_id, $size, false, $attr );
				do_action("end_fetch_multi_{$post_type}_thumbnail_html", $post_id, $post_thumbnail_id, $size);
			} else {
				$html = '';
			}

			if ($link_to_original && $html) {
				$html = sprintf('<a href="%s" rel="lightbox-gallery">%s</a>', wp_get_attachment_url($post_thumbnail_id), $html);
			}

			return apply_filters("{$post_type}_{$thumb_id}_thumbnail_html", $html, $post_id, $post_thumbnail_id, $size, $attr);
		}

I’m assuming my ‘large’ size would need to go in here, I’m just not sure how…

Thank!

http://wordpress.org/extend/plugins/multiple-post-thumbnails/