I have hunted with Mattagammon Wilderness a few times. We started hunting there maybe 30 years ago. That is when Don Dudley owned the store, campground and the guiding business. His son is a Game warden and is on the Tv show about Maine Wardens. He still lives there at the Dam, and if you can try to meet him. He has an amazing life story, moved up to the wild of Maine to raise his family, run a trap line and built that business. He is one of the great Whitetail trackers and a great guy to speak to;
They sold the Business to the Christianson's, and they are also great people. I have hunted with them also. The area is vast wilderness and abuts Baxter state park. Lots of cliffs and caves in that area and great winter ground for bears. If I remember correctly, I don't think I spent to many nights if at all without seeing a bear.
An Adult bear in Maine is 200 Lbs. I have been in Camp there and have seen a Bear taken that was 500. I think for most new hunters black bears are hard to judge size, and usually get excited when they see their first bear and end up taking the first one, hence a lot of bears are taken from 125-200 lb range. If you can judge a bear size and hold out, you should be able to harvest a nice Bear 250+. The bigger bears are there, but when there is a good population of bears, you will see many.
If you flyfish, there is some good trout fishing there, and we used to bring Kayaks and do the river run in the morning fishing all the way down to the flatwater pull out spot. Don would open the dam a bit so we had good water going down. The lake is also a nice place to paddle and see moose along the shore.
You will have a very good hunt, I have been in many bear camps in Maine and I can honestly say they do it right.