Functions that return information about a geometry's dimensions
The ST_Area function and the ST_Length function return information about the dimension of a geometry. For example, ST_Area reports how much area a given geometry covers.
ST_Area
ST_Area takes a geometry and, optionally, a unit as input parameters and returns the area covered by the given geometry in the given unit of measure.
ST_Length
ST_Length takes a linestring or multilinestring and, optionally, a unit as input parameters and returns the length of the given linestring or multilinestring in the given unit of measure.