Even at 6"/150mm the tank is 4 x the length of the fish and 2.66 x the width of the fish, plus 4x the height.
On that terminology it would be like a 2ft golden cobra being in a 8ft x 5.2ft x 4ft tall 1100g or whatever tank....
Your going to the extreme here, is it ideal with that length? No but is it sufficient for someone to keep one as long as they keep up with the water changes, yes definitely.
Channa don't move alot, I don't care what the sites say, flick through all the threads on her and find a 6" fish that has 100l all to its self. Probably count em on one hand.