Size, dimensions, area, extent, magnitude, volume are here compared primarily as terms meaning the amount of space occupied or sometimes of time or energy used by a thing and determinable by measuring.
Size usually refers to things having length, width, and depth or height; it need not imply accurate mathematical measurements but may suggest a mere estimate of these.
Size is also referable to things which cannot be measured in themselves, but can be computed in terms of the number of individuals which comprise them or the amount of space occupied by those individuals.
Since dimension means measurement in a single direction (as the line of length, or breadth, or depth) the plural dimensions, used collectively, is a close synonym of size; in contrast, however, it usually implies accurate measurements that are known or specified.
Area is referable only to things measurable in the two dimensions length and breadth. It is used of plane figures or of plane surfaces (as the ground, a floor, or an arena) and is computed in square measure.
Extent is referable chiefly to things that are measured in one dimension; it may be the length or the breadth, but it is usually thought of as the length. However it is often used as though it were the equivalent of area .
The word is also referable to measured time or to space measured in terms of time; thus, the duration of a thing is the extent of its existence.
Magnitude, largely a mathematical and technical term, may be used in reference to size or two-dimensional extent. It may be used also in reference to something measurable whose exact quantity, extent, or degree may be expressed in mathematical figures; thus, the magnitude of a star is indicated by a number that expresses its relative brightness.
Volume (see also BULK ) is also a technical term; it is used in reference to something that can be measured or considered in terms of cubic measurements; thus, the volume of a solid cylinder is equal to the cubic measure of air it displaces, and that of a hollow one, to the cubic measure of its capacity; two objects that are equal in volume may differ greatly in weight; when a thing expands, it increases in volume .