In this paper, a thermodynamic based constitutive model used to model the behavior of magnetic shape memory alloy (MSMA) during applying strain in an energy harvester. In this type of energy harvester, applying strain changes the internal magnetization of MSMA and as a result changes the flux density around it. Using a coil the flux change can be converted to voltage. In order to study the effect of changing MSMA dimensions on the amount of harvested energy, the demagnetization factor for different dimensions derived from an analytic expression for ferromagnetic prisms and the results are validated by reference data. Increasing MSMA thickness results in increasing longitudinal demagnetization factor and decreasing transversal demagnetization factor. The constitutive model of MSMA is used in modeling an energy harvester using two different configurations; one a pickup coil turned around MSMA and second a system with ferromagnetic core to conduct magnetic flux and the pickup coil around core. Simulation of two models at different thicknesses shows that increasing thickness in system with coil around MSMA results in linear increase of voltage while this parameter in second configuration leads to a nonlinear increase of voltage. Furthermore, simulations show that increase of MSMA width, results in linear increase of output voltage in both configurations but with steepest rate for system with ferromagnetic core. Finally, increasing the length of MSMA specimen shows no changes in voltage for the system with coil around MSMA, while linear increase in voltage for the system with core is recorded.