The influence of particle size on the effective permittivity of a particle–gas mixture in the presence of particle clusters was studied using numerical analysis involving the three-dimensional finite element method. The effective permittivity of the mixture was obtained by calculating the electrostatic energy generated in the computation domain. Numerical results show that for fixed volume fraction of particles in the mixture, the effective permittivity of the mixture increases with decreasing particle size. Static experiments were carried out by using a differential capacitance sensor with parallel plates. The variation of the effective permittivity with particle size is shown by experimental data to agree with the numerical results. The methodology described and the results obtained in this paper may be used to help modify the measurement of particles volume fraction in the presence of particle clusters when a capacitance sensor is used.