Yes, I think it all depends. For example Teddy was born on June 6 2005. He was 4lbs. when we bought him in August 2005. He was about full grown by Thanksgiving, 9lbs. He has been 9lbs ever since!
Our puppy Pippo was born December 23 2008. We bought him at the end of February at 1.5lbs. He just last week reached 3lbs.
So if I had to guess around 6 months is about where they may stay.
A friend of ours has an older yorkie that is around 6lbs and very stocky! As far as the legs growing longer, Im not sure.
