Rumor is that the NEX FF will not be much larger than a current NEX-7. The Sony RX1 is also a good example of how small you can make a fullframe mirroless camera.
In addition, the VG-900 shows that the e-mount will accommodate a 35mm sensor. Mind you, the e-mount lenses are almost universally crap, but I've had good luck using my NEX-7 as a digital back for my M glass.
What is strange in the rumors is that the full-frame NEX is supposed to include antishake as well. Anti-shake and fullframe was supposed to be the combination that couldn't work on the NEX bayonete size.
E-glass has been pretty crappy so far. They're now launching a few good ones. I'm excited about the alpha mirrorless opportunities though. Maybe we'll finally be able to have a SLR-like body that's agnostic to mount.