It may be that at some focal lengths a larger circle (that's in focus) is generated. I'd have to experiment. What's for certain is that the bare minimum circle is 43 mm.

I took a couple of Nikon film bodies to Good Will the other day. They were N-series Auto Focus. My daughter had to shoot film for a couple of weeks in her photography class and I picked them up for that for a few dollars each. I had sold my own film equipment a few years after I started shooting digital. I wish I had kept one. I could have rigged up something to experiment with by cannibalizing one of those bodies.

OK, I hope I never come across as snarky, but I'm sorry if I am.

Everyone seems to be getting hung up on the idea of a round sensor, that was thrown in as an afterthought.

But since you asked, picture this. When your FX lens is projecting an in-focus image on your current DSLR sensor, it is projecting an image that is 43 mm in diameter. This is regardless of whether it's an 800mm monster telephoto or a 11 mm wide angle. It is simple math. We know the image is round and we know it lights the corners of a rectangle that is 36 x 24mm, therefore the circle must be 43mm.

The bodies of the current cameras crops this image. That extra light is actually a problem, it has to be controlled so it doesn't bounce around inside the body. Your shutter only allows a rectangular portion of that circle to "expose" your sensor. Now imagine you redesign that body with a mirror-less sensor so it allows all of that image to hit and be captured. The minimum size and shape of that sensor would be a round array that was 43 mm in diameter. Make sense?

The bottom line is: If your sensor matched the circular image available from the lens when it is in focus, it would give you the most flexibility in creating a RECTANGULAR image. Perhaps I should have advocated the a 43 mm diameter sensor from the beginning, but I was trying to imagine a camera that would have an equal number of megapixels to an FX DSLR camera just for the sake of argument.

My much more serious proposal was for a camera with a 29mm square sensor that would take FX size lenses. I would buy it today. Call it a baby, medium format camera. I personally would shoot with it. That sensor could actually be made a tad larger since a 31.4 mm square is the largest size that can be inscribed in a that 43mm circle of FX image. I would also shoot with a square camera based on the DX family of lenses. I'm not going to figure out what that sensor size would be but it's fairly easy to compute.

There, does that add some clarity?

The real question is why not let the final product (the photo) determine the crop? If one always outputs to "standard" paper or frame sizes....why not something else. Most of my film photography was with Hasselblad 6x6...sometimes full frame but often cropped to best show the subject. When shooting I visualized what the cropping would be. I still prefer square format if given choice but currently mostly shoot D810 Nikon. The original 35mm doesn't match standard paper or frame sizes so often needs cropping unless printed full frame on larger paper size anyway.