What describes the size of a quantity in physics?

Magnitude is a term used in physics to describe the size or quantity of a physical property without regard to its direction. It represents how much of that property is present. For instance, when speaking about force, speed, or other quantities, the magnitude gives us a numerical value indicating how strong or large that quantity is.

Other terms provided in the choices have more specific meanings. Instantaneous value refers to the value of a quantity at a particular instant in time, velocity includes both magnitude and direction of motion, and mass is a specific property that measures the amount of matter in an object. Magnitude encompasses the general idea of size, making it the most suitable choice in this context.