I once wrote a lengthy article for The Online Photographer about image size, which is implicitly also about viewing distance. I compared photographs to paintings. Photography, in the past, has been much constrained by processing limitations. Very large high-resolution images were possible, but quite difficult, and beyond the capabilities of even some famous photographers. That's changed with the digital revolution.
Here's the thing about paintings from the Renaissance onward, and before that, on-the-wall forms like fresco and mosaic: the painter (affluent ones, anyway) were free to choose the size in a way that photographers were not. Larger sizes only involved a bit of extra canvas or board and paint. In the TOP article, I actually went through a large art history survey and averaged out the sizes...I can't remember the details, because this was years ago, but if I remember correctly, the average size for wall-display paintings (and which would generally be famous paintings, since they were in an art history book) was three feet or so. Some paintings, originally intended for museums or palaces, were much larger; but those intended for private homes most often fell into the three-foot range. They were intended to be hung on walls, behind chairs and couches, and they needed to be large enough to be easily seen, but small enough not to be overwhelming in living-room spaces. (See Vermeer.) Because paintings and photographs for home display generally share some viewing characteristics, I think looking at paintings can be a guide for photographers interested in exactly how large their prints should be, if they wish to sell them. I have a dozen or so art photographs in my home, and the small ones are in a (reasonably wide) hallway and arranged like a gallery. You can stand far enough back to get the impact, and if you really want to pixel-peep, you can put your nose against them. Two other larger one, a flower shot by Mapplethorpe and Adams' Moonrise, are in larger rooms and isolated; you can't approach the Mapplethorpe because it's over a stairwell, but for that kind of shot, a six-foot viewing distance restriction is fine -- details aren't too important. For the Moonrise, you need to do both. The Moonrise is, I think, 16x20, and is about as small as I'd want to put in that viewing situation. Any smaller, and it would have to go on the hallway gallery to be well-displayed. I would suggest that people who want to sell their photos for any substantial price must choose the photos carefully, and then consider making them *at least* 24 inches in the longer dimension. For most modern in-home display purposes, with appropriate matting and frame, that would appear to be a substantial piece of art to the viewer, comparable to paintings.
I'd make the point that I'm not arguing that one art form is superior to another, but simply that viewing context (home vs. museum) is important. I think that old style 8x10 or 11x14 B&W prints take very carefully considered viewing spaces...or none at all, and should be placed in a portfolio for occasional viewing. As for Hogloff's argument that some pictures need both long views and close-up viewing, I think he is correct, but only for a certain kind of style. Even with landscapes, there are some that don't need or benefit from close inspection. But, he's correct, others do. The Hudson River-style painting posted by LesPalenik above is one that benefits from providing both, as do older paintings by artists like the first two Breugels...but that is an old and now somewhat unfashionable way of doing things. Maybe its coming back, and maybe not.
