Based on laser-diffraction technique, the maximum entropy technique is applied to measure particle number concentration and size distribution in pneumatic transport. The information of particle number concentration and size distribution can be obtained simultaneously without distort the fluid and the particle trajectories. Detailed information about the distribution of the particle size and concentration in gas-solid two-phase flows near sand-bed surface are obtained in a large-scale wind tunnel test system. The experimental result shows that particle number concentration increases linearly with the increase of the wind speed. The mean particle size decreases exponentially with height (H > 3 cm), but almost unchanged for H < 3 cm. The weakly relationship of fluctuating velocity and particle-size dispersion is deduced from the underlying mechanism of equilibrium transport. This illustrates that transportation close to the mobile bed has selectivity for particle size.