My altums would start paiping in around six months from nickel size, the bodies on the would be 3 1/2" or more. including fins they would be about 10" tall.
My Altums are just approaching 12 inches. They are not up to the challenge of a jumbo tetra yet but they have no problem gulping down feeder guppies. A zoo in Chicago have the bigger Altums in their tank.