If you put it above the fireplace you have 2 problems. Looking up at a screen is not a good idea for extended viewing sessions and it looks like you would be sitting too far away to see the detail on anything but a huge screen.
What are you tying to achieve? Some casual viewing or a home theater experience. I can't tell from the picture how far it is from your seat to the fireplace but just guessing it is about 20 feet. For a total immersion experience it is generally recommended to sit 1.5 to 3 times the screen width away from the screen so you are looking at a minimum of about a 6 foot wide screen. That's about 80 inches diagonal. Above 65 inches flat panel TVs are very expensive so you should consider projectors at that point, but 65 inch TVs aren't that bad with some coming in at less than $3,000. If you want home a theater a better idea would be a projector with a drop down screen between the speakers.