A new stochastic model for suspension/colloidal transport in porous media is developed accounting for the retention mechanism of size exclusion. The medium is modelled by two-pore-size network with triangular pore shape. The micro scale model is established using distribution functions and the macro scale equation system is generalised. The analytical solution is obtained for deep bed filtration of distributed particles in bimodal media. Comparison between model prediction and experimental data shows good agreement, which validates the proposed model.