Hair texture won't change upon cutting, although it often looks different and this fools people.
Buying from a reputable breeder is the best way to ensure size among other things. YTCA members breed to standard, and this means that their puppies will probably be within standard range, which is 7 pounds and below. Since they know their pedigrees so well, they are unlikely to have throwbacks. Breeders that advertise t-cups are much more likely to sell you a dog that isn't standard because they are not adhering to the rules set forth by the YTCA. You also have a better chance of getting a healthy dog with silky coat, good structure, and teeth. The parents are the best indication of what a puppy will look like, always see the parents, and breeder in person. Here's a link to breeders that belong to the YTCA:
Yorkshire Terrier Club of America About The Club