Uncharged star polymers having a porphyrin core and four poly(ethylene glycol)methyl ether (PEGME) arms have been synthesized and investigated (through the study of the structural and dynamical properties) as a function of the molecular mass of the arms, in different thermodynamic conditions. Molecules with short PEGME arms are insoluble in water, whereas in the presence of long PEGME chains, the star polymers are water-soluble; for the intermediate length of the arms, the building up of large rigid monodisperse aggregates, driven by the diffusion limit aggregation mechanism, has been observed by means of static and dynamic light scattering. These aggregated systems, whose sizes are modulated by changing the pH and triggered by the chain flexibility, can be well described in terms of the DLVO interparticle potential in the frame of the colloidal theory.