In this work we report on an investigation of the effect of the magnetic particles size on the effective macroscopic behavior of magnetorheological elastomers (MREs). MREs are a class of smart materials known for their tunable deformation. They are composite materials which consist of magnetically permeable particles in a non-magnetic polymeric matrix. When subjected to an external magnetic field, MREs respond by changing their stiffness and damping properties accordingly. The property of MRE to change their mechanical properties is widely known as the magnetorheological effect. Several factors significantly influence the magnetorheological effect such as the polymer matrix, particles-volume fraction, properties, and size of the magnetic particles. In this study, using finite element simulation we determine the correlation between the latter and the macroscopic behavior of MREs. Based on continuum formulation theory, the constitutive and geometric properties on the microscale are considered to predict the composite’s macroscopic behavior by means of a computational homogenization. Using COMSOL Multiphysics software, the magnetic and mechanical fields were defined and resolved. For a constant particle-volume fraction (φ=20%) and varying mean particle sizes (Փ=5, 10, 20 and 30 µm), a twodimensional representative volume element (RVE) was developed, and applying periodic boundary conditions the simulations were performed for isotropic (unaligned) and anisotropic (aligned) microstructures. From the results, the particle size is found to have a significant effect in the mechanical response of the MRE materials. More specifically, the magneto-induced strain effect is observed to decrease with increase in particle sizes. Also, increasing the particle sizes, is observed to lead to a linear increase in the inter-particle distance for the aligned MRE when the sample is deliberately configured such that the vertical distance between the particles is kept constant for all the particle sizes.