SHELTIE!
AKA, Shetland Sheepdog, "miniature Collie," "little Lassie," etc. Clean, good in the house, highly trainable, loyal, clownish, playful, energetic, alert, devoted, usually good with kids, cats, other pets. Can be noisy, especially the females. Territorial, not apt to roam. Good traveling companion. About knee-high. Herd instincts. Requires grooming (brushing, DO NOT SHAVE!).
Note: my Gracie keeps coyotes away from the property. She barks more than I would prefer but I love her. She is 9 years old and will probably live to at least 15-16 years in good health. I have had many Shelties over the years, they and Golden retrievers are among my favorite breeds. Labs and Goldens are not guard dogs, though they make great companion and hunting dogs.
I was a professional trainer of basic obedience for over 20 years in NW NM through a local community college and trained hundreds of dogs of many breeds. Although Shelties are one of my personal favorite breeds, I would not recommend them simply out of personal preference for what you are asking. You want a small protective dog. If you are willing to put up with a bit of barking and the grooming requirements, this could be your breed. I highly recommend enrolling in obedience classes not affiliated with a pet store whose main purpose is to sell treats, and to spay/neuter at an early age.
Good luck!