Yorkies come in all sizes, but the breed standard calls for yorkies to be in the 3 to 7 lb range. Many yorkies, however, get to be 8 and even 9 lbs. This is very common, esp as they age. Even 10 lbs is not unheard of in a yorkie.
If a yorkie gets to be over 10 lbs, he or she is either too fat or came from lines of very large yorkies. Most of my yorkie puppies grow to be between 4 and 7 lbs. Occasionally I have one that is teeny tiny, under 4 lbs and on occasion, I produce one that is over 8 lbs.
I try to be honest with my yorkie puppy buyers. A breeder cannot predict the future, but she can make her best and most educated guess on the size that your pup will be as an adult. Most yorkie breeders know that you cannot GUARANTEE a yorkie to be a certain size. 90% of the time, if you double the 12 to 14 week weight, you will come within a pound of the predicted adult size. But this is not a tried and true method all the time. Occasionally, one will fool us, and we will see a TWO pound difference. And even rarer, we will see a larger difference than that.
I will do my best to provide you with the size yorkie you are looking for, but I cannot GUARANTEE adult size. I can only give you my most educated guess.