Concerning print sizes viewing distances etc it depends very much on what context the image is shown and what the image content is.
A portrait showing a face does not need very high resolution, there's little interest in the small details (pores in the skin), and you want to see the whole face at once. So 10 megapixels or so is probably more than enough for any print size.
Another extreme would be a wide panorama print (say 3:1 format) of a landscape view, mountains in the distance, a village in a valley, roads with cars, trees etc, a large print framed on the wall at eye height. Then it is natural to walk up close to look at small details, and you can look at the image one part at a time.
You don't need large views from mountain tops though, pretty much any landscape picture can apart from being viewed as a whole also be appreciated up close, let the eye wander around in the picture. A high res print is like a hifi audio recording, the artistic content (picture / music) does not get better, but the quality of the medium still adds value.