The majority of TV’s mounted above a fireplace are 55”.  The best way to determine the right size is to do a size comparison.  As an example, find the specs of a 55” and 65” TV and use blue painter’s tape to outline the shape of each TV.  If putting the tape for each one on drywall, step back and you’ll notice what looks best.  You might have to use a cardboard box cutout if mounting above a fireplace with stone as the tape may not stick.  We’ve installed an 80” above a fireplace but the ceiling was 30’ high so each room’s size and shape will dictate the size of the TV.