I have seen and touched a 5 x 5 x 5 Rubik cube device. It was
allegedly a prototype by Meffert, the people who created the
Megaminx. It was about the same size as a 4 x 4 x 4, with
slightly smaller cubies. It had never been devirginized, nor
did its owner want it to be. The face coverings are of the
same material as the coverings of the Megaminx. No clue as to
how it operates, although looking at the black surface exposed
by partial twists, there seemed to be new techniques involved, of
protuberances coming down from cubies sliding behind other
cubies, at least. Although stated to be a prototype, it seemed
completely developed, and I would expect to see it available
soon.