Final answer:
Eye liner is used to define and emphasize the shape and size of the eyes.
Step-by-step explanation:
The makeup product used to define and emphasize the shape and size of the eyes is eye liner.
Eye liner is applied around the contours of the eyes to create different aesthetic effects. It can come in the form of a pencil, a gel, or a liquid and is available in various colors.
For example, a black eye liner can be used to create a bold and defined look, while a brown or gray eye liner can create a softer and more natural look.