the shots in the gallery you posted look to me as if they were shot with medium format -- 120 film. For yur homemade camera, don't be afraid to experiment -- make another pinhole -- for small format it is more important to have a very small hole and a very round one, so experiment with perhaps drilling one in aluminum from a soda can and filing the edges so it is smooth. Use a needle for a drill and be patient.

It is also very difficult to have too much black tape on a homemade camera, and make sure you paint the inside matt black so there are no reflections. It will take a bit of experimentation, but you'll improve.