without vignetting is ±30.5mm x 30.5mm.
Why do we care about vignetting at capture? Why not have a 43mm square sensor, and just accept major vignetting at capture, but you've got the entire
image circle. You can then decide how to crop in post. You'd never need to rotate your camera, and so no need for vertical grip. Yes, it would be a bit of a waste of silicon, but so what? There could also be camera settings to pre-select format: 3:2, 2:3, 1:1, 4:3, 16:9, etc. If Canon (or Nikon, or whoever) want to stretch their lens lineup for pros, that's how to do it.
** Note: I've made this suggestion several times; some have noted that there might be problems with mirror clearance. I accept that as a legitimate question, but still consider it worth consideration, especially when (not if) we get to the point of mirrorless pro-level bodies.