Magnetic nanofluid (MNF) is one special kind of nanofluid which possesses both magnetic and fluidic properties. Nowadays, investigations of MNF-based hyperthermia heating are emerging as a new frontier in studies of cancer therapy. This work aims to investigate the influence of cluster's size on hyperthermia heating the MNF of colloidal clusters of densely-packed Fe₃O₄ nanoparticles at low field intensity. Emulsion droplet solvent evaporation method was used to assembly oleic acid modified Fe₃O₄ particles into compact clusters stabilized by surfactant in water. Both experimental and simulation works were conducted to study the dependence of the heating efficiency on the cluster's size. In comparison with isolated particles, a more efficient heating could be expected only when the clusters' sizes are small enough that the clusters' shape anisotropy was enhanced. The shape anisotropy is reduced as increasing the clusters' sizes, since the clusters become more and more like spheres. Consequently, the inter-particle dipole interactions change to impair the heating efficiency. When the clusters are totally isotropic in shape, the heating efficiency keeps lower than that of isolated particles despite the cluster's size, although the efficiency can rebound by somewhat at a particular size.