This is off of the YTCA website. Although we may not all be in to "showing" this club is a good example of the ethical treatment of these precious animals.
All breeders may occasionally have an unusually small Yorkie (hopefully healthy),
though no responsible breeder breeds for this trait. Many breeders prefer a
general weight range of 4-7 four pounds believing that size retains desired Toy
qualities while maintaining optimum health. The Yorkie Standard states weight
"must not exceed seven pounds" and as a prospective pet owner you should
realize that even at 7 pounds, the Yorkie is still a small dog. (Females weighing
less than 5 pounds are considered by most breeders to be unsuitable for breeding.)
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http://www.ytca.org/ethics.html learn more about ethical breeding at this link