Self-assembly offers the possibility to organize molecules in a given architecture through a subtle interplay between different noncovalent interactions. Although the kind of molecular association can often be predicted from information present in the individual molecules, the synthesis of supramolecular assemblies having a perfectly defined size and shape remains challenging. Here, we introduce the use of Coulombic interactions to control the supramolecular synthesis of finite, well-defined nanostructures. In particular, we demonstrate that the energy associated with the separation of ion pairs can regulate very precisely guanosine self-assembly into discrete G-quadruplexes. Assemblies comprising 8, 12, 16 or 24 guanosine molecules can be selectively and quantitatively obtained simply by tuning the stabilization of the dissociated anions in the solvent environment. Thus, factors such as solvent polarity, the nature of the anion and the cation-anion distance are shown to have a decisive role in the growth of G-quadruplexes.