A design object's size is its absolute or relative physical dimensions on an X and Y axis. Images and graphic marks are often measured in inches or picas. Print fonts are typically measured in points; online fonts are often scaled according to a relative unit called an em.

A design object's horizontal and vertical size can be manipulated independently or jointly to preserve a particular ratio. Different design objects' comparative size strongly communicates their relative importance within the document. This informational hierarchy creates an order and contributes to a document's balance.