kwmullet
I used the pinhole calculator on mrpinhole.com
I want to use a banquet format (7x17 is my dream camera) I ended up using 8x20 because I had a packet of 16x20 ilford paper I could use immediately to create these paper negs. I punched in the focal length I wanted to use (I love wide angle - both for the field of view and the compactness) I ended up deciding on 12"/305mm focal length. punched all that in and it gave me an optimum pinhole size of .75mm so I used some thin aluminum I got at a hobby shop and gently punched the pinhole, and after some sweat and blood (yep I bled a little) ...
voila.