12"-14" on average but they grow terribly slow. I had one years ago and it topped out at 10". Got him at 7" kept him for 3 years and he never grew past 10". I had to sell him because he eventually became too aggressive to keep with my earth eaters and stingrays. It looks like the tank mates you have will be just fine with him though. I would just suggest to get a large piece of wood for him to hide under. In the wild they tend to live in the tangled masses of trees that have fallen into the water. Often if they are kept with large over bearing fish without a proper place of refuge they can become stressed.
Thank you all for the reply. The pike is about 9-10" right now and has been chomping down pellets like crazy.
JM, he doesn't seem stressed that all. In fact he is roaming in the tank like it is his tank now. Every now and then he'll would go chomp on my 16" kelberi then run away as soon as the kelberi turn looking to bonk at him back.