Size required is dictated by a: bb shell width and b: chainset used, since they can sit on the spindle differently depending on the design.
So for example an FC-2450 chainset like what's on my winter bike, mounted on a cartridge BB with a spindle of 113 would locate the midpoint between the chainrings at 43.5 which is the recommended Shimano road double measurement.
(
drimble he'd been white, he'd been black, Sun 13 Aug 2017, 1:03,
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