Diner66 at 800

This is a scan of a 4x5 inch transparency at 800 pixels wide. This would become only 68 mm wide when printed at 300 dpi.
However, the image contains much more information. The right foot of the woman showing all pixels available in the scan:


This does not look really sharp, because the scanner (an Epson 4870) has not extracted all information from the original image.
However, when printed at 300 dpi, this would become only 25 mm wide and would be critically sharp to the human eye:

rightfoot small

The whole image at this scale would be 1 meter 30 wide, which is sufficient for most exhibitions...
Print sizes vary with the contents of the image. For an overview: Print Sizes

For comparisons between film formats and digital cameras: Film Formats