Taken from here: http://oikos.org.uk/2011/09/tech-notes-using-resized-images-in-wordpress-galleries-and-lightboxes/

Tell WordPress media gallery image links to open the large version of the image instead of the full size.

function oikos_get_attachment_link_filter( $content, $post_id, $size, $permalink ) {

 // Only do this if we're getting the file URL
 if (! $permalink) {
 // This returns an array of (url, width, height)
 $image = wp_get_attachment_image_src( $post_id, 'large' );
 $new_content = preg_replace('/href=\'(.*?)\'/', 'href=\'' . $image[0] . '\'', $content );
 return $new_content;
 } else {
 return $content;
 }
}

add_filter('wp_get_attachment_link', 'oikos_get_attachment_link_filter', 10, 4);