Flat Panel Over the Fireplace
I always recommend against mounting a flat panel above a fireplace—no so much because of heat, because the flue insulates the wall. And unless you have a glass or metal radiator in front of the fireplace, there's no heat rising right in front of the TV. (Of course, if you do have a radiator in front of the fireplace, the TV could get way too hot for its own good!)
Rather, it's because this type of mounting is way too high for optimum viewing, as you suspect. In fact, it's too high for comfortable viewing—you must crane your neck to watch, and you'll likely get sore before a 2-hour movie is over. So if at all possible, I strongly recommend that you find another solution that puts the middle of the screen at seated eye height.
If that's simply not possible, get a mount that tilts the screen downward toward the sofa, though that still isn't great. A much better option is called ComfortVu (seen above), a motorized wall-mount kit that swings the TV out and down to a more reasonable viewing height in front of the fireplace. It can accommodate TVs measuring 55 to 80 inches diagonally and weighing up to 200 pounds, but it ain't cheap at $3600.
As for the TV itself, the optimum diagonal screen size for a seating distance of 12 feet is 90 to 120 inches, but your available space is 69 inches wide and 51 inches tall, so the biggest screen that will fit is less than 80 inches diagonally. How much less depends on the size of the TV's bezel, so let's say 65 or 70 inches, which are the largest standard flat-panel screen sizes under 80 inches. All the major brands offer 65-inch flat panels, but only Sharp goes up to 70 and even 80 inches.
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