First, we must understand what diffusion is! Diffusion is the movement of particles from a high concentration to a lower concentration, in order to reach equilibrium. Cells in particular need this to maintain homeostasis, or an internal equilibrium (stability).
Do you mean molecular size or composition, perhaps? Bigger particles are harder to move than smaller molecules. Additionally, physical constitution of a particle will affect the rate of diffusion. Think of a mouse squeezing under a door as compared to a rock, of the same size. A mouse will more easily be able to squeeze through the gap