This paper presents a model able to relate the frequency response of the multi-resonant circuit of Chipless RFID tags with the spiral resonator structure length. This model is computed from simulated data that was previously validated by comparison with measured data from a printed multi-resonant circuit. The model is validated by designing a two bit multi-resonant circuit using the proposed model and comparing the measured frequency behaviour with the expected one. In this work, the existence of multiple resonant frequencies per resonant spiral is highlighted as well as its implication to the tag design.