Are you sure you want a mirror on that wall? If I were eating at your dining table I would not want to see myself.

It doesn’t appear that seated diners, below the level of the mirror, will see themselves. They are likely to see reflection of the chandelier. This one ( from houzz) is only 40” tall so not quite large enough.

Looking at your picture of the room I immediately saw column shapes. The tall curtains, tall door, the vertical art, the shape of the fireplace and the size and shape of the potential mirror. It seems to me that you need to break up that repeat and pick a different shape mirror or stack the art different, more horizontal. 4 art pieces ,2 on top and 2 bottom rather than a single 3 stack.

A key guide to choosing a size for your mirror is to follow the 'rule of thirds' whereby you take the height of your mantel piece, double it, and you have the ideal height that the top of your mirror should be hung at.

The width of the mirror should never be wider than the mantelpiece or fireplace.

The mirror will be mounted high enough so that no one will be able to watch themselves when siting at the dining room table.

I think floor mirrors from a source like Perigold or Anthropologie might work. A bit larger than you posted would also be better

This is the Jade mirror from Anthropologie with the antique glass. I couldn't get a straight image of it so I had to fudge it to get it on the wall.