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After reading the comments, I've noticed a point that is missing from the other comments. We like to measure things relative to other things. Therefore we should use a unit of measure which you can compare the entire range of expected values for that question simply.
For example how far away is my nearest town centre? 1km. How far away is the nearest city? 10km. How far is it across the country? 500km, How far is it across the continent? 5,000km. How far is it around the equator? 40,000km.
By using all km in this case it's easy to get an idea of the relative distances. But you wouldn't measure your height as 0.0018km. Just my own thoughts!
That's what unit prefixes are for... you can measure your height in cm
Exactly!
You measure your height in metres as in 1.8m or 180cm and for little things it's mm (millimetres) or cm if it's less then 1m.
It scales alot easier then the US measurement system of anything other then the simple metric system.