Clusters of magnetic nanocrystals are very advantageous if keeping the superparamagnetic state of the individual nanocrystals while taking advantage of a large total magnetic moment, as very convenient in sensing, catalysis and bio-related applications. Herein, we demonstrate how the molecular weight of poly(ethylene glycol) exerts a dominant role in controlling the final size of the clusters and individual crystallites forming them, which delineate the final magnetic properties and their potential applications.