I'm not a big fan of mirrors that reflect ceilings. However, if it would fit with your decor, you could age the mirrored surface so that you could get the sparkle without a specific reflection.

I agree with the others that the angle is too high. The center of a TV should be in the middle of your seated head level view. Depending on the technology of your display if you get too far from that center view the quality of the image degrades. You also need to aware of operating temperature range of the TV which will be far exceeded when the fireplace is in operation.

Dear sdifeo, please don't be offended, but IdaClaire and others are so right. The giant clock creates this weird focal point. It's all you see at first and the room looks like little shrubs underneath. When you like something, it doesn't mean it fits in your house. Learned that myself the hard way. I'd like to compliment you with your gorgeous dining table. Take that as your best piece and work from there. I do not understand your choice of chairs around it though. In itself the mirror is the right size. But you have to be sure it doesn't lean back or forward in order not to get a weird reflection of the ceiling or floor. And, to be honest (which will help you most), I believe it is not up to the standard the dining table is setting. Looks a bit cheap next to it. Since your windows also mirror quite well, to me it becomes a bit much in the evenings. So I agree with Sabrina Alfin, that a tall narrow sculpture or vase would probably be best.

Take the decor items off the mantle. They are too small to be effective. Switch the square art near the window, with the mirror. The square art is squished in that space and will look, I think, great over the fireplace. The mirror near the window will reflect both the view outside, and reflect light into the room. Remove the long rectangular art to the left of the book shelf. Too many contrasting rectangles everywhere in this room, including the blinds. You need to break that up. I see 2 round pieces of wall art, one on the wall near the dining table, and one on the floor. Can we see them in better close up? It's possible that the round one would be even better than the square art, over the fireplace. Is the fireplace wood or?