Bill, here is the big issue that comes to mind. As you know, a pinhole's optimum diameter for max sharpness is *very* wavelength dependent, and the 'effective focal length' is likewise very wavelength dependent. If you care about sharpness and want to explore the limits of that, then you might come up with a nifty way to get sharp colour pinhole images by colour sep; I've had some ideas along these lines. This would be interesting because it would not be possible with normal colour films e.g. multilayered films. You'd need to hold the film (or the pinhole) at different distances for the different sep wavelengths... or have different sized holes for the different colours. And then figure out how to bring the colours together in good registry (at an enlarger, I suppose).

Anyway, that nutty thought aside, there is no limit on the pinhole diam, but obviously the bellows or whatever starts to get very long. If you have a big diameter hole and you put the film close to it, you won't get much focus/collimation at all. I think if you just peruse the online pinhole calculators you'll see that big holes mean big path lengths.