There are no certainties about any breed of cat becoming what you consider large. I, personally would consider an 18 lb cat pretty large. We have a neutered male, Maine Coon that will be two years old this coming April. He is about 15 lbs and still probably has some growing to do as supposedly the larger breeds grow until they are about 24 to 36 months old. He isn't fat, but is extremely long and not near as solid as some other breeds are. That being said, we free-feed our cats a quality, very high protein dry food and supplement with occasional feedings of a quality wet food. If you have ever been to a cat show, there are both larger and smaller of all breeds of cats. I would say that it is more of a genetic thing for one to be exceptionally large, but would probably make an educated guess that a male would become larger than a female. I have also heard that the younger a male cat is neutered that it allows him to grow larger but have no evidence to back this up....its just something that I was told. We have a neutered male Egyptian Mau that is as solid of a cat, proportionally as I have ever seen. He is about 14 lbs and shouldn't get any larger. He is just solid muscle and doesn't look like he weighs as much as he does. It really flips the vet out when we take him in and they weigh him and then re-weigh him again, thinking something is wrong with the scales. He's just solid. My only concern with any of our cats is that they are not overweight and that they are not mis-proportioned. A lot of things depends on a cats adult weight, genetics, bone structure, health, and the type of food that you feed can also determine their overall health. I would just do a search on breeders and call them and talk with them about the "general" size of their kittens when they are fully grown. The $600 price range is about what you can expect to pay from most reputable breeders. It wouldn't be impossible to find one cheaper, but I would sure check them out very well. In most cases, you get what you pay for. I also noticed that you had another post about a heart murmur in a Maine Coon. This is also a genetic thing in the Maine Coon breed as well as some other breeds. A good breeder will do their best to not breed either a male or female that has any of this defect in their bloodline.....as it is easily passed on. As far as a murmur goes, sometimes it goes away as the kitten grows.......sometimes it doesn't.....just as in humans.