CHIHUAHUAS: Chihuahuas are among the smallest dogs. Here, a miniature Chihuahau and Jack Rusell terrier cross, Tom Thumb, sits in a teacup. Tom Thumb measures less than four inches from the tip of his nose to his tail and only weighs three ounces. He was born in late July 2009 and is a contender for the title of World's Smallest Dog. Based on the size of Tom's paws and body proportions, his owner does not expect the dog to grow more than an inch or two more.