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There are many standard widths for 120 camera frames: 645, 6x6, 6x7, 6x8, 6x9, 6x12, and 6x17. These numbers are ostensibly in centimeters although in practice a 6x6 camera such as a Hasselblad will expose a 56 x 56 mm frame.
My personal choice
I like 6x6. I find that I take better photographs when I park the camera on a tripod, look down onto the ground glass, and evaluate the composition as though I were looking at a finished photo.