Commercial breeders can breed runts and reduce the size of any toy breed, at the expense of the dog's health, but Chihuahuas are the smallest breed by all AKC, CK, or CKC standards..
there are two breeds that are around the same size.
Prazsky Krysavik which is from outside the czeck republic
it ranges from 2-6 lbs and ranges from 6-9 inches in height
also
the chihuahua which is about the same but from mexico
The Chihuahua is the world’s smallest breed of dog and is named for the Chihuahua region of Mexico where it is believed that visitors first found this small dog and brought it to America.
The normal size for a Chihuahua is around 12 to 15 inches tall and does not weigh more than 3.3 to 6.6 pounds.