Most of their height and length will be done at 9 months old. Some can grow up until they are 1 year old, though. Then after that they will fill out/gain weight until they reach about 18 months to 2 years old. When you are with your Chi everyday you don't notice the growth as much as say someone else would if they saw them months later.
I have 4, and they all grew at the same rate, and reached their max growth at 2 years old.