The smallest iris diaphragm in any of my lenses has a minimum diameter of maybe 1mm and has only four blades. It would be appropriate for about 1 meter focal length. With careful construction, an adjustable three- or four-sided pinhole can be made with a much smaller minimum aperture. A quick check of three blades of about .0012" thick hard material (stainless steel?) suggests a minimum length of maybe .010" for each side of the triangle. Such an adjustable aperture with two fixed and one moveable blade should be fairly easy to make. A four blade adjustable aperture could have a much smaller minimum size, but would require higher precision in construction. These apertures would have more diffraction than a round aperture of equivalent f/number. Since pinholes cost nothing to make and can be made in any size, spending money or time on adjustable pinholes seems more like a fun project than an investment.