The optimum size for pinholes is discussed in the lens FAQ, which can
be accessed via the URL
ftp://butler.hpl.hp.com/jacobson/photo/lensFAQ.
Q37: What is the optimum aperture for a pinhole camera?
A. d = .036 sqrt(Si), where d is the diameter of the pinhole in
millimeters and Si is the distance from the pinhole to the film in
millimeters.
The optimum aperture for a pinhole camera depends on what criteria is
used. The formula given maximizes the spatial frequency at which the
MTF for 555nm light will be 20%.
-- David Jacobson