Here's a breakdown:

* Velocity is a vector quantity, meaning it has both magnitude (size) and direction. For example, "50 km/h north" describes both the speed (50 km/h) and direction (north).

* Speed is the magnitude (size) of velocity. It tells you how fast something is moving, but not in what direction.

In simpler terms:

* Imagine you are driving a car. You're traveling at 60 mph. That's your speed.

* Now imagine you are driving 60 mph east. That's your velocity, because it includes both your speed and direction.

So, the size of velocity is just the speed.