Hi Camilo,
the tracker does have a limit for proximity. It’s due to the optics inside. For the REX the operating distance between eye tracker and eye is specified as 40–90 cm and it’s roughly the same for the EyeX controller. That’s a decent size range and of course we hope that it will be sufficient for most users.
A smaller screen will have near-optimal eye tracking performance everywhere as opposed to a large screen where the performance will degrade the further you get from the tracker.