In this paper, miniaturization of an elliptic-response open-loop resonator filter and design flexibility using similarity transformation of the coupling matrix are proposed. Moreover, the filter with wider fractional bandwidth is designed by the proposed method. In order to verify the proposed method, three 4th-order elliptic-response open-loop resonator filters with a relative bandwidth of 4 % at the center frequency of 2.0 GHz are designed. One is realized with constant-width microstrip line resonator and the others are implemented with different-width microstrip line resonator. Compared with the former one, the latter have shown the size reduction of 13 % and 25 %, respectively. Since it may not be possible to implement the resonators with very narrow spacing for the required coupling coefficient filters with two different configurations representing same response characteristic through similarity transformation of the coupling matrix are proposed. From this design flexibility, a filter with a relative bandwidth of 8 % at the center frequency of 2.0 GHz is designed with realizable design parameters.