The international unit for lengths is the meter. However, larger distances - as in this case - are often expressed in kilometers instead.
The millimetre.
The best unit is a litre.
A decimetre is a metric unit for measuring distance. A diameter is a characteristic of a circle or similar shapes.
The mathematical equation which describes how to measure the distance from Earth to the moon using Earth's diameter as a unit of measure is d = Dcot(p/2)/2 Where d is the distance from Earth to the moon, D is the diameter of the Earth and p is the angle of parallax subtended at moon by the diameter of the Earth.
The best unit for measuring the length of a pencil is inches.
Gram
centimetres
inch
pounds
A millimetre.
kilograms
For a desk like one used in schools the best unit would be pounds. Good luck on your measuring!