There is one unit of length that is not arbitrary.
And it is already in use for measurements across the universe.
It is called the light year.
Somewhat more practical, and arguably more ISO standard like, is the
light second.
At 299 792.458 kilometers it has some resonance in everyday life, like your car may cover that distance in its lifetime.
A light microsecond would be a tad shy of 300 meters and a light nanosecond would be
30 cm long, just under a foot, but still bigger than my feet actually.
We can have a light footed approach to this all
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2011 Honda Insight + HID, LEDs, tiny PV panel, extra brake pad return springs, neutral wheel alignment, 44/42 PSI (air), PHEV light (inop), tightened wheel nut.
lifetime FE over 0.2 Gigameter or 0.13 Megamile.
For confirmation go to people just like you.
For education go to people unlike yourself.