I think it is fair to say that a "reasonable" viewing distance is about the same as the width of the picture, with one feet / 30 cm being the closest, that is a postcard is not viewed any closer than an A4. With this viewing distance the full picture can be viewed at once comfortably. Of course, very large panoramic prints may invite the viewer to step close to look at a part of the picture at a time.
I'm just learning about fine art print-making but have done some observations in high quality offset printing. The highest quality offset prints used in photo books swallow 400 ppi. At book-reading distance a 400 ppi photograph with this type of print quality gives the sense that details are smaller than the eye can resolve.
Lowering the ppi to 300 or slightly below makes noticable difference, perhaps not so much that it seems that resolvable detail is lacking but more a sense that surfaces look less natural due to the lack of micro detail. At ~250 ppi the surfaces can look "like plastic".
So this tells me that 400 ppi is probably good, but rather not lower. This is for books at book-reading distance, which can be about A3 in size when opened up, perhaps a bit larger. Let's make it simple and say 24x18 inches, which yields 9600x7200 in 4:3 format which is 69 megapixels.
If you make the print larger, so will the viewing distance (if the viewer is supposed to see the whole image at once), so you will get away with those 69 megapixels regardless of size. More pixels gives you some ability to crop. I think framed pictures behind glass also are a bit less critical, so you can get away with inkjet prints despite that they do not resolve as much as 400 ppi.
This is no exact science of course, but it is clear to me that current DSLRs of ~25 megapixels is a bit too low to match the possible print and eye resolution. However, when DSLRs reach 40 megapixels, which I think they will quite soon (40-50 megapixels is a suitable "limit" for 36x24mm sensor with the current lens resolving power at least) I think many that use medium format today will think DSLRs are "good enough" in terms of image quality. Say with 45 megapixel DSLR you have the same pixel size as current APS-C cameras and you would get 8250x5500, which is 20.6x13.8 inches at 400 ppi (52x35 cm) which would at least be good enough for a spread in books of common sizes. Lowering to inkjet standards were ~300 ppi is considered good enough, those 45 megapixels may be "all that is needed" for any size. But we're not there yet, and still there are advantages of the larger sensor area (gathering more light), and of course there's more to a sensor and readout electronics than only megapixels.