Given the aspects of the visual field (figure/ground, motion, gradient), descriptors are structures that describe the interrelationship of the figures within the visual field. A given descriptor can refer to figure/ground, motion, or a gradient, though obviously the application will differ. It is possible to use size to create a figure/ground relationship, such as a grid of squares with one much smaller square. It is possible to use size to create motion, such as a square getting larger then smaller over time. And it is possible to use size to create a gradient, such as a row of ten squares, each one getting slightly smaller.
Related Concept: Continuity/Discontinuity