I adopted one once and I had only a thirty gallon tank at the time.  Someone got tired of keeping fish and was going to feed them to her cat.  So I brought them home.  He was huge!  He was way too big for the 30 gallon tank I had.  I had a lid on the tank but he was so strong that he would actually hit the top of the lid so hard that he would knock it off with a "BANG" and jump out of the tank!  I would say he was a foot long at least or more!

Yes, he was small when my friend got him and she had him in a 55 gallon tank.  Her tank was leaking so she got rid of it and gave me the fish instead of to her cat.  She only had 4 fish left and they were all huge but the high fin shark was the biggest.  She had had the fish for quite a few years before she gave them to me.
I should also say that 1 of the 4 fish she gave to me was a silver common goldfish.  I separated him from the other three since they were all tropicals and put him in a 10 gallon tank.  Then he went out in my pond in the spring.