It depends on the lines. Some stop growing at 6 months, others continue until 12 months. You should ask the breeder how quickly their lines tend to mature. A responsible, reputably breeder should be able to give you a pretty good estimate.
Who knows...I thought Bella was done growing a long time ago (shes three), but I switched her food a few months ago and she has gained weight (im so glad she did, she needed too). I think all you can really do is make sure they get the nutrients they need and are healthy, the rest is just genetics. I think as far as height and length, Bella was finished growing by the time she was 12 months.