6x4.5. I picked up a pair of A16 backs for the Hasselblads on a whim a couple of years ago and have rarely used them. While the 35mm transparencies paled beside the "super-slides," I still prefer the slightly larger square transparencies of the 6x6. I love the 6x6 format: the challenge of seeing/shooting/printing (full-frame) in an unconventional format (at least in the opinion of those who argue that pictures are either vertical or horizontal in orientation) were the primary reasons behind my decision to go with the Hasselblad system. Maybe, though, it's time to revisit the 6x4.5...