In this paper, three dimensional finite element method is used for the analysis of thin film bulk acoustic wave resonator (TFBAR) at 5GHz. The TFBAR is placed on thin membrane after removal of substrate material for the suppression of loading effects and three different geometries (rectangular, polygonal and circular) are implemented and modeled. The size of fabricated TFBAR are from 100×100 um2 to 200×200 um2 and full-wave modeled results are compared with the measurement. It is found that the modeled and measured results agree within 1% in terms of series and parallel resonant frequencies. Furthermore, the different shapes of TFBAR revealed slightly different bandwidth characteristics, which is defined on frequency spacing between the series and parallel resonant frequencies. The another goal of this work is to study the variation of the size of resonator on how affects the performance of TFBAR. As the size of resonator incease, the electrical impedance of TFBAR decrease at the resonant frequencies and out of resonant frequencies. These phenomena contribute to improve the effects of the attenuations of TFBAR filter in stop-band to some degree.