Adult teacup Chihuahuas range in size from 2 to 3.5 pounds. This can be subject to argument from breeders, as normally-sized Chihuahuas typically range from 4 to 6 pounds.
The best way to determine whether or not a Chihuahua puppy is a teacup or not is to look at the size of the parents. No female Chihuahua under 4 pounds should be bred, but males can and should be smaller. If the female is closer to 4 pounds and the male is very small, there is a higher possibility that the puppies will be teacup-sized. Some disreputable breeders call the runt of the litter a teacup so that they can charge more, but with proper care, these runts often grow to be the normal size for the breed.