This paper studies the effect of the specimen sizes and shapes on the settlement behaviour of sand soil with gypsum from Al-Najaf city in Iraq. The investigation aimed to verify the impact of the studied parameters of the prediction of the settlement using reliability-based analysis. Three sets of soil samples with different shapes and sizes, 60 mm diameter Oedometer cell, 60 x 60 mm box container and 100 x100 mm box container were remoulded with three different densities related to the maximum dry density, 100%, 95% and 92%. The results reveal that different settlement behaviours were determined at several normal stress levels during the soaking process. The samples exhibited reductions in volume due to the applied stresses, while under the same level of normal stress, the final settlement values due to soaking were higher for the samples with a lower value of initial relative compaction. Further, the test results showed that at any initial relative compaction and applied vertical stress, the higher settlement value is achieved by the larger square specimen (100x100)mm.