It is the 44x33 sensor, so not of the same size as 645 in film. Same as the digital Fuji, Hassleblad, et al.
Having the film Pentax 645 swung the decision for me in the Fuji v Pentax choice. Anything else is a big step up expense wise. I have Silkypix that I use for my Lumix S-1 so I can hit...