First of all, I would check out
Yorkshire Terrier club of America to find a good reputable breeder in your area. Once you have found a breeder they will help guide you with finding the perfect and healthy Yorkie. I have been taught that you measure from the base of the neck to the base of the tail. Then from the floor to the top of the back. You want a square body. Of course you wont be able to determine this on a small pup. Around 20 weeks should give you accurate measurements. For instance, one of my boys is 10.5'/10.5'. My other boy is 9'/9'...Perfectly square. Good luck with your search...