This paper focuses on evaluating the impact of metal thickness of a microwave coplanar based sensor dedicated to the microwave dielectric spectroscopy of single particles and individual biological cells. A sensitivity study has therefore been achieved for metal thicknesses comprised between 0.3 and 20 µm. After the validation of electromagnetic simulations with measurements of 10 μm-diameter polystyrene bead, both capacitive and conductive contrasts have been defined for the different metal thickness of the sensor. The maximal sensitivity improvement is therefore achieved for a thickness value similar to the diameter of the particle or cell to measure. Capacitive and conductive contrasts are increased by a factor 2.4 and 1.75 respectively. The study leads consequently to an important design and fabrication rule of such a sensor.