Hard to tell. The photo you attached is too close up. Take a step back and photograph the entire wall.

Additional pic

Is that where you will have it--sitting on the mantel? Looking like it will roll off if someone slams a door?

No. No mirror is suited for that location because it has nothing to reflect but a boring ceiling, a fan, and a blank wall. Put mirrors only where they have something interesting to reflect or have a utility (like checking your appearance before leaving the house).

And whatever you put there needs to be asymmetrical, not centered. You need something long, narrow, and with strong or deep colors hanging down the left open area.

I agree with that last paragraph of Tatts. Ideally, I would suggest you get rid of the mirror and replace with a tall, narrow piece of art with strong colors and hang it on the left, not centered. As for the question, is it too big? Sort of, not at exactly? It is the wrong shape for the space. The diameter is ok because a smaller diameter would be too small. The shape does not compliment the fireplace.

I don't think a mirror is suited for that spot. looks blah.

This is what is meant by reflecting back something interesting:

Plus, it's hung, and is styled nicely w/the vases on either side.

try some art work (doesn't have to be a saying, i'm just posting this pic because it's a good size and very similar to your fireplace

or diff mantel styling.

I'd try and chisel off that middle shelf so you could hang a taller picture.

Thank you all for looking and commenting on the post! Although the answers do not specifically answer the question, your given advice is thought provoking. The mirror would be hung, we are not chiseling out the brick design and the mantel will be styled as I mentioned in original question. Appreciate the pictures above!

It is too big and not good for the beautiful fireplace. Art would be a better choice and a few accessories that help soften the brick (real plants in woven containers, for example).

Yes, it's too big in that space.

Maybe a set of mirrors that can 'cascade' down the brick wall and create a shape that works with the brick ledges.