If the fireplace is decorative and does not produce heat or smoke, I think a 70-in TV would be the largest I would place there. You might find that is a bit larger than you are comfortable with for the viewing distance, so you may want to measure off the distance from fireplace to where one would be viewing the TV from, and go to a brick-and-mortar store and view various sizes from that viewing distance. (I paced off the distance at home, and paced off the distance at the store before even looking at the screen when I purchased my first large TV. My friend thought the TV would be too large, but it worked out great while the TV lasted, and its replacement is just a few inches larger.)
If the TV is to be viewed from sitting down, having the TV tilted down a bit and having recliners (or at least seat backs that tilt back) so one can watch the TV with the neck straight and eyes straight forward and seeing the TV straight on would be nice; one doesn't want to get a kink in the neck just because of the viewing angle.
If the fireplace is a live fireplace that produces heat, I'm not so sure I would want to use the fireplace to bake sensitive electronics.