The convention is chosen such that cations lie at the center of coordination scheme, with anions residing as nearest neighbors. Relative sizes can be expressed at the radius ratio. When coordinating identically sized spheres there are several possible ways of packing so as to create contact between the spheres. The most efficient way to pack together a layer of spheres is through Hexagonal Closest packing. Note in the figure below that a hexagonal planer lattice can describe the atom locations.
Coordination is defined by the polyhedron of anions that form about a cation.
The total strength of valencey bonds that reach a cation from all neighboring anions must equal the charge of the cation. When all bonds are of equal strength then this is termed isodesmic . In many cases the bond strengths are not all equal. If you have a small compact group comprising a highly charged cation and less strongly charged anions then their e.v.'s will be large. When the bond strength is exactly one half the bonding energy of the anion bonds are termed mesodesmic .