too tiny and not too large... "just right" in the words of Goldilocks... Even doing a pair of medium sized pieced would work, or layering them one slightly in front of the other could also work. Obviously, everyone who has a fireplace mantel will be dealing with varied sizes of wall space, ceiling heights... choose the best fit for your home. Once your artwork is up, be sure not to block it with tall items, taller items will go to the sides so the artwork can be seen. Smaller items can be layered in front of the artwork.
Decorating works best is odd numbers... so try to work in grouping of three or five.
Here, I added candle sticks for height and a tall oil lamp on each side. These flank the artwork, farming it and not getting in the way of seeing the art either. Three items on each side...similar in color, a little silver and glass to add shine--on both sides
About TVs above the mantle... I know lots of folks put the TV above the fireplace... I considered it too... I don't love it there for two reasons... I don't normally look that high to watch TV, so it doesn't work for me in that respect. Secondly, I think it is really hard to make a TV look stylish in general...on the mantle it's out there, there is no hiding it. If it's on, it's colorful, loud and busy as the focal point or if it's off, it becomes a large dark box...both of which are hard to get right in the décor department... So for those reasons I kept our TV out of sight in a piece of furniture. It's a really tough decorating dilemma...