how big a pinhole you need really depends on the look you are after, and the focal lenght of the pinhole.

What you need to remember is that the wider you go, the more likely you are to get the bed of the camera in your photo. What some people have done in the past is to create 2 lens boards - onewith a large pinhole, and one with the actual pinhole you will use.

the larger one will create a very fuzzy, but bright enough image for you to see what you will be getting. Once you're happy lock the camera off and replace the lensboard with your shooting pinhole and take the photo

For information I can recommend and of Eric Renner's books on pinhole photography..