We have 4 Florida Gars in a tank 8ft x 2ft x 2ft. Ours are with a Mbu puffer, and a Perruno Catfish. They are currently around 16" long and have not grown recently, giving us the impression they are nearly fully grown.
We bought these fish from Richard Hardwick, the expert who writes for PFK Magazine, he says he has seen them upto 2ft long but in 35 years has never seen them any bigger.
That said, they are better off in groups, like most Gars,and a group of 4 fish all 2ft long will need a considerable tank.
Ours are soon to be upgraded to a tank 8 x 3 x 2 .Ours eat very well and take whitebait and prawn as soon as i put it in the tank.
Although their bodies bend they are generally stiff bodied and need room to manoever around the tank. Eventually they will go in a tank 14ft long, 5ft high and 4ft wide if they grow big enough.
Great fish if you can provide the right tank and tankmates, not sure about an Arowana with a Florida Gar though, Arowanas can be very aggresive!!
Clare