The angular resolution of the fovea of the human eye is one arc minute. Having an angular resolution of your display panel higher than one arc minute thus very quickly yields diminishing return (and only because of lack of proper filtering in the image processing parts).
This means that for any given diagonal size/resolution you have a viewing distance, beyond which you're basically wasting money if you opt for the higher resolution panel.
For 50" 1366x768 panel this distance is:
dist= 50"*(16/sqrt(16*16+9*9)) / (1366 * tan (1/60)) = 109,6" = 9,1'