if it is a clown loach (tropical/freshwater), you do not need to worry about size yet, they take years to put weight on. Ive had mine from about 2cm now at 5" but that has taken years to get there, albeit like with any fish, you have to consider their growth in relation to the size of the tank.
or are you referring to a clown fish (marine) this is like as in finding nemo, the disney film? if so, do you have a marine tank, or a tropical?