Scale and Proportion: When referring to an artwork's size we use the term scale; the size of the art object in relation to another object, most commonly to the human body (life-sized, miniature, enormous).
Proportion refers to the size of parts of a whole or elements within the object relative to each other. Ex: proportions in terms of size relationships within the human body.
Michelangelo’s David demonstrates both scale and proportion. Scale, in its size relative to the people viewing it (larger than life). Proportion, in the relative size of human characteristics within the body.
Contrast: “the arrangement of opposite elements (light vs. dark colors, rough vs. smooth textures, large vs. small shapes, etc.) in a piece so as to create visual interest, excitement and drama.”
Both images contrast between shadows/dark areas and well-lit/light areas (value).
(http://www.wetcanvas.com/forums/showthread.php?t=1348826)
Internal references: Using an object (most commonly the human) that is familiar (know the size of) to put into perspective the relative size of an artwork and/or its component.
We get a sense of how small the tree house by comparing to broccoli, which is easy to imagine the size of.
Golden Mean: The perfect combination of balance and harmony based on the ratio 1:1.618.
The theory suggests that humans are naturally trained to recognize this pattern, so it is an effective tool to determine relative sizes, where to place detail, or how to organize a layout or scene. The eye is naturally drawn to the center of the spiral, for example. It can relate to the entire scene or just components, however some argue that the golden ratio is naturally produced regardless of its acknowledgement.