A 12 inch with 22 being "normal" shouldn't be wide enough to cause a major problem with angles and size relationships being odd -- it'd be like a 21 mm on 35 mm film, approximately, decidedly wide but no fisheye. It will introduce considerable light falloff. You can calculate how much pretty easily -- use geometry to calculate the light path length from pinhole to corners, and then apply inverse square law to get a ratio of light level; each factor of 2x in light level is one stop, and fall off of one stop is quite noticeable, while two stops makes an image that's hard to print or reproduce.
One of the wonders of pinholes is they can cover 120 degrees, if you can control the light fall off. Another is they keep everything in the same sharpness from 2-3 times the pinhole distance, out to the horizon. But the longer the focal length, the slower the pinhole (if you stay with something close to optimum, for which I use 1/25 of the square root of "focal" length, measured in millimeters), yet the fuzzier the image on the same size film, because the pinhole gets bigger as you move it away from the film, if you keep the optimum size. With huge film, however, you won't be enlarging, so you'll get a sharper print than you would with a "telephoto" pinhole on small film.
Oh, yeah, one other thing to be aware of -- if you make a pinhole camera from a round can with the pinhole at the surface of the can, you'll get reflection lines (parallel with the can axis) in the image. If you make it with the pinhole at the center of curvature (essentially a half round), you'll get both more even exposure and no reflection lines (but be very sure to thoroughly blacken the inside of the can and the film side of the pinhole itself, since light from the hole will reflect back to a line through the hole and parallel with the can axis, causing flare).
