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I've had several, both indoors as living Christmas trees, and also outdoors.
They live over a hundred years. They can grow (in California, where I live) to over 100 feet high. In older neighborhoods here, they are often the tallest tree in the area.
The problem is that they can only take a little bit of frost. Even here, you don't see them more than 20-30 miles inland, because we get frosts here if you're not close to the water. An older, established tree would be able to stand more frost than a young tree, but as your tree grows, you won't be able to protect it anymore. It appears to be happy where you planted it, but I imagine you have a frost danger where you live.
It's like any other tropical plant in northern Florida: there's a risk you'll lose it in a really cold winter. But you can't keep a Norfolk Island pine small, except indoors.
Good luck,
Dr. Jim Sperber