Size is relative to the viewer and distance. Structure describes the form of an object. An object is identifiable by its shape and components, reduced to the level where it is minimally identifiable. Structures are building blocks. Objects can be identified by their shape and size through our senses.
Minerals are the building blocks of the earth.
Plants and animals scale and adapt their structures to their environment. Crystals have characteristic structures. Organisms are built up from cells. All matter can be reduced to atomic particles. Sampling allows estimations of great magnitude.