The current study aims to elucidate the effect of solid particle size and concentration on the background gas phase velocity in a monodisperse and bidisperse gas-solid flow. The 2D simulations are performed using a compressible multiphase CFD solver based on a Eulerian approach which comprises of AUSM⁺-up flux functions for both the gas and solid phases. To perform the assessment, a supersonic under-expanded jet injected into quiescent atmosphere at sonic velocity and higher pressure than the atmosphere is considered. The combination of particle size and volume fraction that results in lower range of Stokes number results in significant reduction in maximum magnitude and location of occurrence of the gas phase vertical velocity relative to a pure gas and highlighting the effect of introduction of solid particles in a pure gas. In a bidisperse mixture, the gas vertical velocity is found to be proportionately lying in between the respective monodisperse mixture of finer particle and coarser particle values depending on the relative proportion of finer and coarser particle. A correlation using a parametric form [(p_s1φ_s2/d_s1 + p_s2φ_s2/d_s2)ε_{s total}] is found to correlate the percentage decrease of gas vertical velocity relative to a pure gas for the dispersed mixture.