A comparison of sand-polymer interface strength tested, at a normal stress less than 100 kPa, by varying the sample size in a fixed mode direct shear box test is shown. Four different shear box sizes were used. Two types of cohesionless sand and a smooth HDPE with a thickness of 1.0 mm were employed. It is found that the use of small shear boxes increases the frictional coefficients with decreasing normal stress and that the larger the length of shear box the lower the frictional coefficient at a given normal stress.