The size of a unit of a physical quantity is the amount of the quantity represented by the unit. The sizes of the fundamental units of a system of units are established when the fundamental units are selected, and these sizes determine the sizes of all the derived units in the system. Thus, the size of the units of area and volume depends on the unit of length chosen.
The International System of Units uses special prefixes to form series of decimal multiple and submultiple units of various quantities. The ratio of the size of each decimal multiple or sub-multiple unit to the size of the fundamental unit is a power of ten; for each unit, this power of ten corresponds to the prefix in the name of the unit. Thus, the millimeter is a thousandth of a meter.