The size of the photograph that you want to display is in part determined by how close you will to the photo. For a photo to mimic real life properly, the viewer must be at a distance where the photograph subtends the same angle as "reality" did when the photo was taken, bearing in mind the focal length of the lens. For example, if a 50mm lens on a 35mm camera subtends a horizontal angle of 46 degrees, a photo taken with that lens and printed full size will only be viewed properly if the viewer stands at a distance such that the photo itself subtends 46 degrees of the viewer's vision. Therefore, a very large photograph may be appropriate if the viewer is going to be standing far enough away that the subtended angle is correct. Or a small print may be appropriate if you expect the viewer to be quite close.

Of course, that's a rather rigid set of rules about viewing photographs. No one can deny that having a large photograph may permit the viewer to see details that would not be perceived in a smaller print, and the sheer size may drive home a particular vision of the object -- as in large prints I've seen by Avedon. Alternatively, printing small may be desirable if you wish the viewer to be able to grasp the entire image in one view, without having to shift the eyes and piece together the image as you process it mentally. There are rules, and there are artistic objectives.