[Bayart]

French perspective here : everybody uses centimeters and millimeters are common. I just came from a hair cut and the girl doing it asked for my clipper size in millimeters. I have an intuition, and it's usually pretty close, for how much à millimeter, centimeter, meter and kilometer are. All that to say it's got everything to do with habit and not random size body parts.

[NoImmatureAdHom]

What I'm curious about is whether people raised with the metric system and nothing else are more accurate in estimating e.g. lengths than people raised primarily with Imperial / U.S. customary.

So we do an experiment where we have people look at, and maybe handle, random objects and then ask them to estimate the length. Does imperial / metric make a difference? My guess is no, but I have significant probability weight on yes...maybe 20%. And wouldn't that be cool!

[orwin]

I think you can go look at sociological experiment about crowd estimation, you might find a response: while those experiment are mostly done in the US, they are often replicated in metric countries. (i don't have any clues about the response to be honest)