Decided to switch out a round wrought iiron decorative piece for a round mirror. Just wanted to see an example with a round mirror (28”) i had on hand. i know it is too small for the width, as it shoukd be 2/3 the size of the width if the fireplace. My question is is that the width of the shelf or the width of the fireplace, column to column? Also would welcome mirror suggestions, etc. i kind of like the gold finish. Thank you!