The bigger you go the farther away the viewer has to stand. Viewing Distance. 40"x40" or even more is possible it just has to work with the subject. Pinhole is soft but the softness can work in your favor. You have to try it out furthermore I hate to say it but at bigger enlargement digital can be better than analogue. Unsharp masking etc.. might help in reducing the softness.
As I've said before it has to work with your image. Sugimotos twice infinity images are super soft but they work.