Clown Loach Growth is unpredictable. U can start with a shoal and years later there is significant differences. As for what peeps here have been saying about 5" and up grow real slow afterwards. I have some examples of loaches I've obtained that may help.
*from 1.5" to 3.5" in about nine to ten months (petsmart)
*from 2" to 5" 3 years from previous owner in a 10 gal.
*from 7" to 8" in about one year or a little less
*from 2" to 9" Qty. 5 shoal were obtained by original owner 5 years ago. Had 5 total and the other 3 were 5"-6", so major size and thickness differences too.
I can say the 12+ or so that I got between 4" to 7" from the start have grown about half to an inch in about a year.