It is difficult to tell from pictures alone. Study the muzzle length, the head shape and then make comparisons such as this site shows:
http://silkyterrierclubofamerica.org/comparison.htm Size alone can't make the determination. Just on this site, you will see tiny under-2 pound Yorkies as well as some up in the teens. From hearing the owners describe their babies, it seems the most common characteristic is the loving nature of the breed.
Good luck on your selection! Buy from a reputable breeder. There are many here on YT. If you ask for a general area, someone will probably have a good referral for you if you decide on a Yorkshire Terrier. I don't think you could ever go wrong with a Yorkie!
One more note -- the dog in your picture has her ears down. If you want that look, do not shave the puppies ears. Look for puppies with a heavier ear, let them stay full of hair and they will be likely to droop. It is not the breed standard -- but many love the look.