I also think that the 49x37mm sensor is the optimal size of sensor to have... I use a Hasselblad CF-39MS back on Contax 645 camera and I find the crop ideal as to maximise the performance of the lenses I use as it crops out exactly the part of the lens that needs to be cropped, especially if one uses the lenses at wider aperture settings... That said, I wouldn't be surprised at all if Hasselblad comes out with an intermediate size (exclusive to them) sensor to the two Sony Cmos sensors... I wouldn't consider the full size 54X40.5 sensor at all for three reasons...
1. The size of it would many times force my to stop down the lenses and thus make my system slower, but would limit the shallow DOF too if it is needed,
2.The huge file sizes that I will have to deal with for each and every capture,
3. I'm now converting a Nikkor 24mm PCE lens to use on the Contax (with full dedication of electronic aperture control), I find the AOV ideal for ultra WA, wider than that would be too wide, but more than that, its image circle is just enough for my current sensor (very sharp too), on a 54x40.4 sensor it causes strong vignetting...