Well my memory is bad. The 4x5 was 16x20 and as I recall I did a magnification test of 20x24 on 8x10. I have the 16x20 here next to me -- from about 18" away it can be described as silky. From about half that distance you can see grain. I would think proper viewing distance to be no less than 2 or 3 feet.
I posted it to the gallery section: