They say great things come in small packages. Well, if that is true, then this tiny dog is really, really great. The 4-year-old Chihuahua was recognized by Guinness as the world’s smallest full-grown dog. Chihuahuas are among the tiniest dog breeds in the world, but a Chihuahua this small is definitely not something you see every day. At less than 4 inches tall, Milly is smaller than a cat and could easily fit in a pocket. But don’t be fooled by her size. Her owners say Milly has a big personality to compensate for her tiny frame.