I have two yorkies. 4.5 lbs and 5.5 lbs. They are both the exact same size and you can't even tell the difference. They are both TINY! anything under 4.5lbs is super tiny, fragile and personally more of a "toy" then a dog.
If your worried about getting a yorkie that will be too big, you need to find a reputable breeder and know the stock well, or get an adult that is your desired size.