The optimal image size is up to you. For me the bigger the negative the
better. The smallest pinhole camera I use is 4"x5". As for the size of the
pinhole, there are some formulas; they depend upon the pinhole to film plane
distance. They can be found on several web sites. If you do an Alta Vista
search for "pinhole" you can find them. As a general rule that I have found,
the smaller the hole the sharper the image. I have several that are around
f/1024 the produce very sharp images, but the exposures are quite long. The
beauty of this type of photography, is that you never know exactly what the
image will look like when you are done.
David