I have seen one grow to about 18 " in a 55gallon tank- one things I have noticed through my experience is they tend to grow somewhat slowly. I currently have a baby that I bouhgt not quite two years ago, it was nose to tail maybe 2.5 inches, and now it's pushing 4 inches. It eats everything I drop in there, frozen or dry food, loves all it's caves, glass tunnels, plants and to swim upside down whenever in the mood.
I have had some that when put into a tank with tetras the tetras tend to lose thier eyeballs at night, and small things like neons or guppies might disappear. Other than that, they seem to not be very fierce predators, and are very safe for general community tanks, minus small tetras.
Most of the ones I have seen that are more than 12 inches are more than 10 years old.
They are electric- and terretorial towards other electric fish- I am amazed that the elephant nose and the black ghost are getting along in the pictures posted here.