
I keep big dogs, as well as Chi's and Yorkies. I agree with Marianne about total supervision. We had someone on CD a year or two ago, absolutely devastated that their Great Dane had broken the Chi's neck in play.
My Beardies are used for socialising the Chihuahua puppies, and they are never frightened of large dogs. I was doing ringcraft with a 3lbs Chi pup, and would purposely stand next to the lady with the most enormous Rottie male that I had ever seen in my life. I honestly believe it is what stops toys from being snappy, yappy little things. Yappy dogs ( at seeing large dogs) are really only scared dogs.
I have photos all over the house of Chihuahuas asleep on top of Collies. But the little ones are crated at night for safety. My Beardies have been supervised with the little ones, and know what is expected, but they are still supervised.
However, I once had the unfortunate experience of witnessing a Border Collie whip round the neck of a Yorkie too fast. He was only playing the same as he would play with a larger dog. Snapped her neck, and she died in my arms. Don't ever, ever think you can keep both large and small dogs together without extreme vigilance. I never want to hold a dying dog in my arms again that has lost life in this way.