Why not a 21.6mm x 21.6mm sensor?

MatsP wrote:

as Anders have suggested (and I hinted in another post). With that size of the sensor you could shoot 4/3 portrait and landscape, 17,3x13,0, and also 1/1 15,4x15,4 within the m4/3 image circle. The 1/1 shots would have slightly bigger area than the 4/3 shots. Theoretically you could use the whole sensor area 17,3x17,3 when shooting 1/1 with an adapted APS or FF lens and get significantly bigger area, comparable to the APS-C sensor area. Only a little smaller. Maybe also some m4/3 lenses have big enough image circles, I don't know (Sigma 19 and 30 should have it as they also exist in APS-C versions for NEX).

I like the 1/1 format and find myself cropping to that quite often.