A 30 inch round mirror would be attractive but . .. are you sure a mirror is your best move? What will it reflect? Unless it reflects the outdoors . . imho art is usually a better choice to signal your color palette and add personality and color.

The fireplace is opposite a large picture window to our backyard.

If you use candles, a proper overmantle mirror (with a ledge at the bottom that sits directly on the matlepiece) will look good, as whatever height candles you put on there will be reflected. For me the worst mistakes in overmantels are to have it wider than the shelf, or much too small. The best ones match the width or are only slightly smaller. The fireplace is so simple that you could either match its simplicity or do something quite dramatic here. I agree with Bamemory that rectangular mirrors look best above a fireplace, but also the overmantels that are square across the bottom and arched at the top look great, they can really add height to a room.

Here are two pix of mine.

well, I am trying to attach this. I am not good at this.

One more. These are both Uttermost mirrors, but I have several that are not.

If you are still looking for a mirror above your fireplace I attached a mirror made by Uttermost. We have a large collection of Uttermost mirrors. Here is the link to our mirrors on our store http://www.essentialsinside.com/index.php?main_page=index&cPath=48_192

Here is the information for the mirror that I attached here:
Dorigrass Framed Wall Mirror (42"H x 32"W) by Uttermost, available at the essentials inside, www.essentialsinside.com

This framed wall mirror features hand forged metal finished in distressed mocha brown with black undertones and golden brown highlights. Mirror has a generous 1 1/4" bevel.
Details:
Height: 42 in.
Width: 32 in.
Depth: 0.5 in.
Mirror/Glass Height: 30 in.
Mirror/Glass Width: 19.75 in.
Mirror/Glass Depth: 0.187 in.
Weight: 43 lbs.

We have free shipping and no sales tax
Marika