This article focuses on the physical chemical aspects of the formation of supramolecular nanoparticles with defined size and varying shape through electrostatic self-assembly of macroions and multivalent aromatic counterions. For cationic poly(amidoamine) dendrimers and different di- and trivalent sulfonate groups carrying azo dyes, the onset of interdendrimer connection and assembly size (hydrodynamic radius 20 nm < R-h < 150 nm) depend on counterion/macroion loading ratio. Centrifugation and dialysis experiments show assemblies coexisting with individual dye-loaded dendrimers with lower dye/dendrimer ratio at small loading ratio, while around charge stoichiometry only assemblies are present. Zeta-potential measurements reveal a positive charge for samples with excess dendrimer. For excess dye, overloading to negatively charged assemblies is possible for some dyes, which is consistent with concentration-dependent stability revealing a second mode of more loosely bound dye ions. Kinetic versus thermodynamic effects are discussed based on varying the preparation route. The interaction enthalpy is an important factor in determining assembly size. Solution structures are characterized by static and dynamic light scattering, while atomic force microscopy showed that assemblies can also be deposited on surfaces.