6x7 is almost twice the size of 645, so it's the way I'd choose too - but was a little confused by your original question with which way you wanted to go. Bronica also made a 6x7 system, and there's a really great rangefinder - the Mamiya 7 (also 7II). That particular rangefinder is still selling for a pretty high price used, but most everything else in MF can be had for a song.