I was thinking about more about resolution than screen size. 2560 x 1600 has been the highest 16-9 resolution for a long time now.
The next wide screen resolutions would be,
2880 x 1800
3360 x 2160
3840 x 2400
5120 x 3200
But I seem to remember 5000 x 3000 is about as much as a human eye can resolve, but i could be wrong about that.
I would pay a boat load of cash for a monitor with a resolution of 5120 x 3200.