Hello,
You can put darn near anything in any size tank. The question is how humane is it?
Example:
I have a friend who used to have 1 large green moray eel in a 55 gallon tank. The eel, when fully elongated and swimming the tank was over 50% of the tank's length. So the thing sat around and swam in circles, hid under some huge coral rocks, etc. He maintained that water VERY well so it was always healthy.
The eel grew more, and my buddy wound up trading it off to a pet store where it went into a 125 gallon, then was sold.
One of my favorite marine fish within the hobby is the Panther Grouper. My other is a Red Emporer Snapper. Back when I had saltwater fish I could never afford more than a 38 gallon, or so, tank (that whole setup is sitting in my garage). I took my grouper back to the pet store when he (or she?) got too big, and I kept my Snapper. My Snapper's growth was severely stunted, but, in talking to my wife about him, I think he lived well over five years in that tank.
Point being, you can keep whatever you want, but eventually it will not be healthy for the fish, or even if in perfect health, you will have to get the fish adopted by a pet store or facility that can manage it. Most predatorial, meat eating, saltwater fish grow pretty quick.
Me personally? I wouldn't want to be trying to extract a big Lionfish out of my tank. I would look at Dwarf Lionfish... not quite as flamboyant in overall expansion of the fins, but equally as striking in color, if not more.
Best Regards,
Ssjrr family