You know how game manuals start off by giving you some health-related advice? Most commonly, you'll get advice like:
"Take a break every hour for around 10-15 minutes"
"Play in a well-lit room"
"Do not play when you're tired"

Simple enough.

However, some manuals for older games like Earthworm Jim 3D and DarkStone give one more advice that I don't understand:
"Play on as small a screen as possible"

Can someone explain to me why this was a recommendation? It's not necessarily about improving game performance; it seems to be a recommendation to avoid eye-strain or something like that. However, I used Google to find info on small vs big screens and couldn't find anything about smaller screens being better for your health. The opposite is implied more often.

Probably old wives tales I'd guess. Bright screens in dark rooms are definitely rough on the eyes (use a bias light!) but I don't think size would matter much, and also size relates to distance anyway. Staring at a small phone screen in a dark room honestly strains my eyes way more than my monitor or TV.

Possibly to hide their naturally limited resolutions, since I imagine neither game to scale very well.

Besides, who'd want to look at a game like EWJ3D all big?

I suspect this was more aimed at games with a first-person perspective (driving games initially, then FPS' when Wolfenstein/Doom arrived) where a larger screen allowed players to become more immersed in a game - and possibly more subject to nausea when presented with fast moving scenery.

As StingingVelvet has said, there seems to have been little actual data to back this up so presumably a case of "Cover Your Ass" syndrome.

These old games ar often made in 320x200 (or 320x240) and look like shit when zoomed in. These days we call it retro, but back then it was the state of the art.
Also scrolling is an issue. While specialized graphics hardware on the Amiga for example allowed very smooth scrolling, CGA and EGA did not and even VGA took several years to come up with a more or less smooth solution. The games would always stutter (compare Alien Breed on the Amiga with Alien Breed VGA) which can be very stressfull for the eyes, the larger the displayed area is. Well, I do :) We played the original Burger Time on a 83" screen without smoothing the image.

As AstralWanderer mentioned, some people seemed to become nauseous from playing Doom or Descent on a big screen, but I am unsure whether Earthworm Jim 3D or Darkstone could really have a similar effect to those people... Maybe the publishers just wanted to add it just in case.

I am not sure if epileptic people are also more easily affected by bigger screens.

I've heard that it's better to not use full screen when watching videos with lots of flashing (like that one Electric Soldier Porygon excerpt that's been known to cause seizures in people who don't normally get them).

On the other hand, one person complained that they couldn't play False Skies because of the flashing, even though the flashing was only in a small part of the screen (specifically, the highlighted entry in the current menu). (Also worth noting that this was changed in a patch; the flashing is now gone, unless you go into the settings and re-enable it.)

These are all parts of the standard epilepsy warnings. The small screen one is less common, but generally, it was thought (may still be thought - I don't suffer from epilepsy so can't comment) that larger screens led to a greater chance of seizure. This was especially true with CRTs.

The PS3 medical disclaimer noted that large screen televisions should be avoided and that users who suffer from epilepsy or photosensitivity should use the smallest screens possible:

https://legaldoc.dl.playstation.net/ps3-eula/psn/u/u_health_en.html

Could probably see justification for games that are very low res and don't have an option to go up.

Interesting to see it in EWJ3D though. That game, even nowadays, scales without issues on high resolutions (recently played through it in 1920x1440).

Thanks guys, interesting answers.
I don't think this sort of notice is around anymore though one user mentioned it was on the PS3 manual/leaflet, so even 10ish years ago that warning still existed.

Was asking since I play on an Ultrawide, was worried that I might be killing myself.

No worries, you will survive. It's a leftover from CRT days mainly, with larger screens outputting way more UV light your way, not to mention greater seizure potential for photosensitive people. Not good for kids sitting too close to the screen playing games, burning their retinas away either.

Anyway, the high-end crt pc monitors had awesome colours and scaled wonderfully with low res input thanks to their fine shadow mask (compared to crt TVs, or the abhorrent scaling of lcds). LCDs caught up in the colour department only in recent years for most part. For a cheap stopgap measure that is easy to mass-produce, somehow it doesn't want to go away.

I seem to remember seeing some advice that contradicts this: Specifically, apparently you should use a big screen but give yourself some substantial distance between you and the screen while playing or watching.

Yeah, I think Spectrum_Legacy cleared that up, it's probably a CRT thing.
Nowadays you hear stories about how bigger screens are much better than a small phone screen.

Possibly unrelated, but it may be based on how fast the motion is, turning camera angle or elevation. Afterall they probably were programming it on not only low resolutions, but 14 inch screens were the most common probably.

Unrelated, i started playing Primal (PS2 game), which i was playing on a 40 inch or something TV back in 2015. Now the game was fine, until i was going up some stairs. The motion of going up the stairs and the jumpiness was enough to get me nausea, on par with car-sickness. Were i to play it on normal size or a smaller screen I'm sure i wouldn't have had that issue.

A related query on "simulator sickness" can be found on the first letter at:

Dan's Data: Atomic I/O letters column #25