centimeter
The metric unit of length is the 'meter'.
The basic metric unit of measure for length is the meter (m).
You could use either centimeters or millimeters, but probably centimeters.
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.
An average pencil when new would be around 15 centimetres long.
Centimetre
The base unit for length is the metre.
millimetres
The metric unit is the meter scaled to centimeters.
Millimeters.
metre
In the metric system, meters are the base unit to measure length.