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@gbj_tester
I have actually used 140mm cranks on my lo-pro by way of experiment, dropping straight down from my usual 175s. Adaptation was very quick, and adjusting the gear to give the same gain ratio allowed me to go exactly as fast as on the long cranks/big gear set up. If you read the literature on the effect of changing crank length, this will not come as any surprise.
Since, by common consent, changing your crank length wont make you faster (or slower), the only reason to do it is either a: your range of joint movement constrains the upper limit of crank length (this applies to all of us eventually, but few of us are so constrained at common crank lengths up to 175), b: some geometrical feature of your bicycle or riding style impels you to reduce the crank length to gain clearance somewhere, or c: you're particularly concerned with the finer points of aerodynamics on a recumbulent™ or triathlon bike and think shorter cranks will enable a reduced frontal aspect.