Anyway, roughly speaking, at enlargements bigger than ~1:1 (contact print), the lack of detail will become evident. But that doesn't mean that the image falls apart.... if the method is suited to the subject, you can go much larger.
I think you hit it on the head Keith. There's no answer to this question; it just depends on whether the image is suitable for such a big display, taking into account the appearance the pinhole capture method will lend to it.