A box brownie would have a plastic meniscus lens. If you just take the lens off a camera, there's a lot of light getting in, but no means of focusing it, so you get a blur.
If you imagine a very small aperture on a lens, the light is just going through the very centre of the lens (we'll assume
really small), and if you recall any optics you'll remember that if you start drawing diagrams of how a lens forms an image, the light through the centre goes straight through without bending. In this case, the lens isn't refracting (bending) the light, so doesn't need to be there. That's the principle of a pinhole.
There are a number of different formulae to calculate the optimum size of a pinhole, which depends on (amongst other things) the distance from pinhole to recording/imaging plane (supplied on request

). With a large throw (measured in feet or yards) the pinhole could be made with a half inch diameter pin...
I demonstrate a camera obscura with an 18 foot drop; the image (from an f/18 lens) isn't very bright, but about 1000 times brighter than it would be from the optimum pinhole for this 18 foot distance. Say about f/984.*
* Other formulae and calculations are available.