i would say it depends on temp and how much you feed as that will dictate how fast he grows.
i would say that 200 would be an absolute minimum just for the fact that when you put a fish like a clown knife is a smaller tank you dont get to see much natural behavior. he would most likely be sedentary most of the time, and for me seeing my clown knife glide through the tank is the number one reason i have him.
My 17" CK has some crazy swimming abilities. He likes to flip upside down and scratch the hump on his head on the sand. It's crazy how they can manuver on a dime, darting foward, back, and upside down!
Mine is in a 150g. I am going to build a tank this spring/summer in the area of 500g, just to be safe.