Design principles are guidelines used to arrange elements within a composition effectively. The question asks for the principle related to the "relationship between different elements ... and their relative size."
Let's define the options:
(a) Rhythm: In design, rhythm is created by the repetition or alternation of elements, often with defined intervals between them. It creates a sense of movement, pattern, or visual flow.
(b) Proportion: Proportion refers to the relative size and scale of the various elements in a design to each other and to the whole. It's about the comparative relationship of dimensions (size, quantity, degree). Good proportion creates a sense of harmony and balance. Examples include the Golden Ratio or classical orders of architecture which have defined proportional systems.
(c) Harmony: Harmony is achieved when all elements in a composition work together cohesively to create a pleasing and unified whole. It results from a successful combination of other principles like balance, proportion, rhythm, and unity. It's an overall quality, not a specific relationship of size.
(d) Symmetry: Symmetry refers to a formal type of balance where elements are arranged equally on either side of a central axis (bilateral symmetry) or around a central point (radial symmetry). It creates a sense of order and stability. While size is involved in symmetrical arrangements (e.g., identical elements mirrored), proportion deals more broadly with relative sizes, even in asymmetrical compositions.
The design principle that specifically refers to the relationship between different elements in a composition and their relative size is Proportion. Scale is also closely related, referring to the size of an element in relation to a standard or to other elements (often including human scale).
\[ \boxed{\text{Proportion}} \]