FWIW, I've seen pinhole images done on 8x10 film with a focal length under 30 mm, which cover the whole frame (albeit with light fall-off in the corners like a Holga, but still printable and with detail recorded over the whole frame). The field of view limitation for a well-made pinhole is more likely to be from vignetting by parts of the camera than by light fall-off; most negative film will let you overexpose the center by one to two stops, and underexpose the edge by one stop, and still produce an image that just needs a little dodging and burning; with reduced EI, low-contrast development, darkroom manipulation and the right film (like 400TX), you can get away with still one more stop of difference, which would be equivalent to a 4:1 path length difference between center and corners of the frame.

On 6x9 cm, you could probably get away with a 35 mm pinhole, if the camera were carefully made to avoid vignetting. 45 mm isn't at all uncommon, and though the exposure will vary too much over the 6x9 frame for slide film, not many folks shoot 6x9 slides anyway.