Changing Gallery images size?

Use the shortcode_atts_gallery filter to override the size of the images used before making your call to get_post_gallery_images.

add_filter('shortcode_atts_gallery','force_large_images',10,3);
function force_large_images($out, $pairs, $atts) {
  $out['size'] = 'large';
  return $out;
}

Make sure to remove the filter when you have your data, or all your galleries will always use large images.

remove_filter('shortcode_atts_gallery','force_large_images',10,3);