The question asks which among the given options is not one of the Gestalt psychology laws of perception. Let's explore each option to understand the concept behind the Gestalt laws:

1. Closure: This is one of the Gestalt laws of perception. It states that our brains tend to fill in gaps to create a complete, whole object. For example, if we see a circle with parts missing, our brain tends to perceive it as a complete circle.
2. Continuity: This is also a law of Gestalt perception. It suggests that elements arranged on a line or curve are perceived to be more related than elements not on the line or curve. This describes our tendency to see continuous patterns.
3. Proximity: Another law of Gestalt perception, it states that objects that are close to each other tend to be grouped together. The spatial proximity of objects can determine how we organize visual input.
4. Relative size: This option is not among the Gestalt laws of perception. Relative size is a concept in visual perception that refers to the perception of the size of one object relative to another. It is more of a depth cue rather than a Gestalt principle.

From the above analysis, the correct answer is: Relative size.

Gestalt psychology focuses on how people perceive visual components as organized patterns or wholes instead of many different parts. It explains some cognitive behaviors related to visual perception and is often applied to understand human behaviors and cognitive experiences. However, "Relative size" relates to relative depth perception and size constancy in different contexts, not part of Gestalt principles.