The ISO length unit to use would be the cementer.
I would use centimetres.
I would use inches.
You would use the SI unit meter (m) to measure the length of a DVD case.
The length of an aeroplane would be measured in metres.
It would be normal to use either meters or feet, but you can use any unit for length (inches, cm, mm, km, etc.)
A meter.
miles or kilomiters
You would use meters.
I would use feet
i would use inches
I would use feet.