Original: The magnitude of a mathematical object is its size: a property by which it can be larger or smaller than other objects of the same kind; in technical terms, an ordering of the class of objects to which it belongs.
Simplified: The magnitude of a mathematical object is its size: a property by which it can be larger or smaller than other objects of the same kind. In mathematical language one would say: It is an ordering of the class of objects to which it belongs.