If you are geek minded like some of us,,,,maybe not me,,,you can find a fascinating discussion (With cartoons) of how to determine correct pinhole size from an engineers perspective at:
You have to download the .pdf file to see it. It is for guys with calculator holsters, but maybe the rest of us can glean something from it.
As far as I know there are two age old formulae for determining pinhole size.
.0073 times the square root of the focal length (I use this one and it works well for focal legths near the same length as the diagonal of the film).
The other is Lord Rayleighs formula 1.9 times the square root of the focal length times the wavelength of light you want to bring to focus at that focal length.
But then, why not just go to :
and let the computer do all those nast calculations for you?