Don't beat yourself up, but vow to be your dog's advocate. A small dog is going to need protecting from certain situations all his life, so you need to work on ways to do that. You don't have to allow anything that you or your dog don't like or that you see as potentially harmful, and from what you describe, your small puppy was at least bruised and exhausted by the rough play. Just tell your neighbor there's too big a size difference and last time your puppy wouldn't eat or drink afterward, so sorry, no more.