The fundamental understanding of the driving forces in electrostatic self-assembly is highly desirable for the design of novel systems and of more effective synthesis strategies. The focus of this study is the effects of the electrostatic interaction on supramolecular self-assembled nano particles formed by cationic dendrimers as model polyelectrolytes and oppositely charged di- and trivalent dyes, elucidated by changing the solution ionic strength. Increasing ionic strength results in the formation of larger nanoparticles, although, the screened electrostatic interaction of the building blocks may be expected to result in the formation of smaller particles. Yukawa potential and DLVO theory have been used to understand this phenomenon. The screened electrostatic potential decreases the nanoparticle repulsion resulting in larger aggregates, which also causes an increase of the nanoparticle charge leading to stabilization. Contrarily, the ionic strength has no effects on the nanoparticle shape and on the dye stacking due to their pi-pi interaction. This shows how the electrostatic interaction controls the dimensions of the nanoaggregates through the stabilization mechanism, while the secondary interactions, and in particular the pi-pi interaction, encode the nanoparticle shape. Revealing these relationships is a key step in understanding the ionic association of building blocks under secondary interactions.