From what i have read i think it depends on if it is a peruvian or colombian pictus catfish. Peruvian get to about 6 inches and colombian get to about 10 inches. I am not sure where i got the info from, but i will do some google searches and see if i can come up with the website.
EDIT:
Have found a little info but not exactly what im looking for. Make sure that the kind of pictus catfish you are talking about is the Pimelodus pictus as they are the smaller pictus catfish and only get to about 6 inches, but frequently get 4-5 inches