You could use either centimeters or millimeters, but probably centimeters.
An average pencil when new would be around 15 centimetres long.
Centimetre
Any metric unit would do. But it might be easier to pick something smaller than the length of the pencil, like a centimeter, or a millimeter.
a ruler!
Millimeters.
Metres
You would measure it in centimeters.
A ruler would do the trick.
It's gonna be inches/centimeters.
to be honest I would use a metre ruler that really helps a lot place your metre ruler on your desk. put your pencil starting from 0. Then measure it. This is how you would measure it.
Centimeters
Centimeters