they live in salt, fresh and brackish...
the ones in fresh are usually a brownish silver, the ones in salt are usually a high silver.
And yes they get HUGE, Depending on the food source they would reach 2 1/2ft quiet quickly and then slow down slightly.
I believe they are all born male and at around 50cm? they become female...
If you want to keep one for life, After 3-4yrs you'll need a pond