As others have responded, light, flickering or sudden changes in light to dark can stimulate headaches. Also keeping your eyes focused on a single point for extended periods that are different than your usual focal length can do the same.
With respect to screen size, use your own subjective experience and taste. Going by the numbers is a good place to start, but you may find a smaller screen to be more comfortable if you regularly experience headaches. Room decor can also be a big difference. If a 42" screen is mounted on a dark background, you may get the same immersion as a larger screen on a light-color wall. I recently made this change, and the difference was spectacular.
With respect to screen size, use your own subjective experience and taste. Going by the numbers is a good place to start, but you may find a smaller screen to be more comfortable if you regularly experience headaches. Room decor can also be a big difference. If a 42" screen is mounted on a dark background, you may get the same immersion as a larger screen on a light-color wall. I recently made this change, and the difference was spectacular.