Stonefish grow to about 13" and do it VERY quickly......mine went from 3" to 13" in less than a year and weighs close to 3 POUNDS.
They can be housed in a large tank (mine's in a 300) with any other fish that's too large to eat, just like any other Lionfish or Scorpionfish....they have the added advantage of moving very little and spending most of their time completely buried in the substrate.
Color-wise, they are extremely variable, changing to match their surroundings, which can go from blah to brilliant oranges and yellows if they are housed with faux sponges, etc.
The Stonefish IS the most venomous fish in the world, but keep in mind the spines are defensive weapons only, primarily evolved as a defense against being eaten by sharks.
The stronger venom of the stonefish, compared to many other scorpionfish, is likely due to the Carpet shark that lives in the same area.....one jab in the roof of the mouth as it''s swallowed will get it promptly spat back out and most sharks won't make that same mistake twice!
If you look closely the "rock" in the sand behind the 10" 6-line grouper is a 13" Stonefish...the curved part is its mouth.
Mike