A meter.
centimeters
The metre, the yard or the foot.
I would use centimeters.
Kilometer would be the best unit to measure the length of a car, as the length of a typical car is usually in the range of a few meters, making meters too small a unit for practical measurement.
To measure a car's length, I would try yards or feet. Maybe both. It mostly depends on the legnth of a car. A stretch limo would use yards while a VW Bug would probably be feet. For an average car, though, yards.
metres or feet
The ISO length unit to use would be the cementer.
It's SI unit is meters
I would use inches.
I would use centimetres.
Meter
The length of an aeroplane would be measured in metres.