I'm curious if anyone has any materials from the CRT era which describe recommendations for optimal viewing distance for various TV sizes.

NTSC picture quality is optimum at a distance of about 5 picture heights. However, in most home living rooms, TV was (is?) viewed at about 10 feet distance regardless of picture size.

NTSC picture quality is optimum at a distance of about 5 picture heights. However, in most home living rooms, TV was (is?) viewed at about 10 feet distance regardless of picture size.

So then say for a 13" picture tube, this should be 52 inches. I've also read that for best protection from x rays, we should be sitting at least 6 feet away from a color TV? I wonder if this 6 foot figure applies to small screen TVs, or perhaps is overly conservative.

The amount of X-rays generated by an electron beam is highly dependent on the value of the high voltage. The X-ray scare and the 6-foot suggestion came about with color sets that used defective high voltage regulator tubes with inadequate shielding for those tubes. The defective tubes had the internal shielding elements crooked, allowing X rays to escape. Even then, the concern was for long term effects, not immediate danger, and the industry had a mass campaign to replace defective tubes. The picture tubes themselves have always included lead in the glass to shield against X-rays (or external shielding glass in earlier sets). You may find early sets with warnings to the technician about being exposed to the back of a CRT with an open cabinet.

I think I remember reading about that. So in this case, a 1980s solid state set with a potted flyback, is more likely to be just fine at only a few feet? I've noticed though the 80s sets don't have metal shields around the back of the CRT like some older tube sets do. Would be interesting to actually measure.

Yes, is to be safe for our hobby uses. Of course, for watching, is a relation of it's 480i intrinsic resolution plus the resolving details of the TV circuits and CRT.