Found this on the internet. Sounds like they get darn big! 40 lbs in the wild! I wonder if it's ok to let it go in the wild if it's already in the wild where you live? Fishermen like it cuz it's delicious.
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When it comes to catfish in your backyard pond we are not really stocking either of those extremes, but typically the catfish that go into our ponds and water gardens can get very large, very quickly; sometimes up to 10lbs! In the wild the channel cat can get up to 40+ lbs. By far the most popular catfish that is sold by water garden and pond centers is the channel cat; so for this argument’s sake let’s pick on them.
The channel cat, Ictalurus punctatus, is most often sold to pond owners who have the belief that these fish will somehow keep our ponds cleaner and clearer by eating all the garbage, algae, and fish waste that occurs in the pond. Any experienced, honest pond fish reseller should tell you this will not be the case.
The channel catfish, Ictalurus punctatus, is a fish of “MOSTS”. Ictalurus punctatus is the MOST populous catfish species in North America, it is the MOST popular catfish in aquaculture practices, it is the MOST adaptable catfish living in habitats from clear to muddy waters and lakes to streams, and the MOST popular state fish being the official fish for Iowa, Kansas, Nebraska, Missouri, and Tennessee! It is the MOST popular catfish with U.S. anglers, and the MOST tasty delicious catfish. It is also the MOST popular catfish for ponds, and possibly the MOST misunderstood choice of pond fish on the market.
So let’s dish up the channel cat. The channel cat is an omnivore actually, which by default makes it a scavenger, but the channel cat is not a fish to depend on for housekeeping in your pond. In its natural settings the channel cat will eat “smelly” foods like decomposing fish, but also chow down on worms, crayfish, shrimp, other fish, lots of frogs, insects, larvae, seeds, and algae too. However; the channel cat will actually adapt very well to the feedings that you provide for your expensive koi and pond fish. They will very quickly become the primary consumer of your expensive fish food, easily muscling your koi out of the way at every feeding; and the channel cat can eat a lot! Channel cat in the pond equals skinny koi and high food bills. If the catfish is not satisfied with the feedings you provide it would not be surprising if smaller fish in your pond start to disappear. Blame the heron, but the catfish may not be so innocent either!
Channel cats are also nocturnal, and become active at about the same time that most other pond and water garden fish are trying to settle down for the night. This may create a certain level of stress to the more desirable pond fish like koi and ornamental goldfish. The channel catfish is rarely bought for the purpose of adding beauty to the pond because they don’t have great coloring, typically grey or “gold”. They also hide most of the time only coming out for feedings. And they will not create better water quality or reduce waste in your pond.
I dont think I should let it lose. I dont think it's in the wild like in river though.. it's more like pond or lake? 
