Amendments can control the runoff and soil loss by protecting the soil surface. However, scale effects on runoff and soil loss control have not been considered yet. The present study has been formulated to determine the efficiency of two plot sizes of 6 and 0.25 m² covered by 0.5 kg m⁻² of straw mulch with regard to changing the time to runoff, runoff coefficient, sediment concentration and soil loss under laboratory conditions. The study used a sandy-loam soil taken from summer rangeland, Alborz Mountains, northern Iran, and was conducted under simulated rainfall intensities of 50 and 90 mm h⁻¹ and in three replicates. The results of the study showed that the straw mulch had a more significant effect on reducing the runoff coefficient, sediment concentration and soil loss on a 0.25 m² plot scale. The maximum effectiveness in time to runoff for both the scales was observed at a rainfall intensity of 90 mm h⁻¹. The maximum increasing and decreasing rates in time to runoff and runoff coefficient were observed at a rainfall intensity of 90 mm h⁻¹, with 367.92 and 96.71% for the 0.25 m² plot and 110.10 and 15.08% for the 6 m² plot. The maximum reduction in the runoff coefficient was in the 0.25 m² plot for the two rainfall intensities of 50 and 90 mm h⁻¹, with rates of −89.34 and −96.71%. The maximum change in soil loss at the intensities of both 50 and 90 mm h⁻¹ occurred in the 0.25 m² plot, with 100%, whereas in the 6 m² plot, decreasing rates of soil loss for the intensities of both 50 and 90 mm h⁻¹ were 46.74 and 63.24%, respectively.