I think there are two questions I could break down your question into.
1) What is the mature size of a clown?
Mature clowns can be from 2" (if in a mated pair) up to 6-8"+ (Tomato, Calrkii & Maroon complexes). When talking Occy's, usually a mature female will be 2.5"-3.5".
2) What is the growth rate of a clown?
For the Tomato & Maroon complexes, I have seen about 7/8"-1" per year. I have seen in Occy's the growth rate is variable, but I have experienced 0.5"-1.0" per year.