For best results it depends on the distance between the pinhole and the film. A good free program for determining the pinhole diameter is Much information on pinhole photography and camera improvising is at and Drills small enough for this application are scarce and fragile. Pinholes are often made by slightly penetrating very thin material (usually brass or aluminum) with a needle, sanding off the burr that this produces, and repeating the process until the desired size is reached. It isn't difficult.