What term describes the size of a quantity or how big it is?

The term that describes the size of a quantity or how big it is is "magnitude." In scientific contexts, magnitude refers specifically to the numerical value or size of a measurable quantity without regard to its direction or other characteristics. This can be applied to various physical quantities such as length, mass, and energy.

For instance, when discussing a vector quantity like force, the magnitude is the length of the vector, indicating how strong the force is irrespective of its direction. In contrast, other terms listed—velocity relates to speed in a directional sense, force involves an interaction that causes an object to accelerate, and dimension refers to the physical size and shape of an object in space—do not specifically focus on the concept of size alone. Thus, magnitude is the most appropriate term when referring to the size of a quantity.