It isn't uncommon for a panther to triple its size in a few months of captivity with proper care. He will eventually be over a foot long. That's why they need tanks that hold a couple hundred gallons from the start. Also, larger tanks for predators give more wiggle room for your water parameters since they are messy eaters and big poopers so filtration can be a challenge.
Sadly for the panthers, their adult coloration is less striking. Because they are not valuable to an lfs once they fade to brown and there aren't that many large tanks around to take them, they can be difficult to get rid off when they outgrow the tank. IMO it's one of the fish that gives the hobby a bad name, taking wild animals from their homes to keep in a tiny box until they aren't cute anymore and then flushing them isn't ok.
Hopefully that's not your plan. They are a great pet if you can give them what they need