We have investigated the magnetorelaxometry (MRX) of various Fe3O4 nanoparticle suspensions utilizing a differential fluxgate setup. To estimate the distribution of the hydrodynamic size of the magnetic nanoparticles (MNPs) with organic shell, the cluster moment superposition model is applied. On a sample series of MNPs from the same batch diluted in water-glycerine mixtures of varying viscosity, it is demonstrated that the model is suited for the estimation of the distribution of the hydrodynamic size of MNPs and clusters. Since a MRX measurement lasts only a few seconds, it is also highly suited to study the aggregation kinetics. This is demonstrated on MNPs with silica shell suspended in 5% phosphate-buffered saline solution.