Millimeters.
You should clarify what you want to measure about it: its diameter, mass, weight, color, etc. If you want to measure its diameter, that would usually be specified in microns (= micrometers).
Inches (or centimeters, if you're working in the metric system).
Meter.... or more likely millimeters
Metric ruler
The number of the pencil is based on the type of carbon lead used inside. The thicker the carbon lead gets the higher the number of the pencil becomes. If you notice a mechanical pencil is 0.9mm in diameter so it would be called a #0.9 pencil. A #2 pencil's lead is 2mm in diameter. So Why would we call a pencil a #3 pencil? Because it's lead is 3mm in diameter.
An average pencil when new would be around 15 centimetres long.
Micrometer
milimetersMilliliter
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.
Inches (or centimeters, if you're working in the metric system).
You should clarify what you want to measure about it: its diameter, mass, weight, color, etc. If you want to measure its diameter, that would usually be specified in microns (= micrometers).
Meter.... or more likely millimeters
you would use gallons
You could use either centimeters or millimeters, but probably centimeters.
The width of a pencil would be the metric unit of centimeters
Metric ruler
That would normally be measured in cm. It might also be measured in dm., but this unit is not very common in daily usage.