I would suggest going with black slab marble.

I think you would have to go with black, you need something to tie in with the TV right above, the furniture is grey and the walls are a dark grey, so black would work best.I also think if you can find glass tiles they may give you a less monochromatic look but it would be edgier and modern!

White marble:

Black marble:Dark slate:Variegated slate:

Use large format tile:

There is a company named Porcelanosa that has just the large format tile you are looking for. Modern, European and very sophisticated. It will be much less expensive than marble to both buy and install.
They are located in Spain but many of their products are warehoused in Texas.
I find their website lacking but they have great catalogs and samples. Just call their customer service line.
Good luck!

And if you search Houzz photos for the manufacturer, you'll see plenty of examples. Our local paper just has an article about porcelain slabs, up to 8foot long, 5 foot wide. Maybe a slab look, less seams, would look more modern to you.