In my opinion the mirror is too small

Nice mirror Different for sure !! I always go along with that
But yes also think its too small for the Rule of thumb
But Hay Rules
are meant to be Broken !!!
Have voted for the TOP one

First one, but I would raise and center the mirror. I would take away the vase and place the two candlesticks where that was, with taller candles.

Hang the mirror higher.

I'm sorry, I think they are both too small. The top one would be the best out of the two, but you have to center it on the actual hearth bit of the fireplace I think. It seems off-center, because you probably centered it on the mantle? Doesn't look right to me. And it could be a bit higher up.
If you center it on the hearth, you'd have to tweak the other items to balance it out again. Difficult fireplace design to decorate around. Hmm...

I think the first one looks better, but I agree that a bigger mirror would look better!

The first one. Though I think a larger mirror should be there as well. That is if you are insisting on a mirror. If you are open to other suggestions then a colorful medium to large wall hanging would work a bit better.

First one but hang higher

Neither they are too small for space. Go vertical and go rectangle. 4 ft or taller. In dark frame.