Typically, the formation of clusters (e.g. by gas phase condensation) is a stochastic process, which leads to formation of clusters of different sizes. The cluster size commonly refers to the number of atoms forming the cluster, rather than its linear dimensions. Cluster sizes range from several atoms to hundreds of atoms, and the resulting ensemble of clusters is usually characterised by a rather broad size distribution.

In some systems, clusters of some specific size have increased stability, and, as a result, such clusters are formed is much larger numbers than clusters of other sizes. Such clusters are often referred to as “magical” clusters, as the number of atoms in them is not arbitrary, but is exactly equal to some "magic" number. A classic example of the magic clusters is the family of fullerenes including C₆₀ C₇₀, C₈₄ (see Fig.). The method of mass spectrometry is widely used in studying cluster compounds.