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The influence of the size of microstructural features on the macroscopic mechanical behavior of crystalline materials has been the subject of many studies over past decades. These size effects are controlled by the development of plastic strain gradients in the microstructure. A decrease in characteristic sizes of the microstructure may lead to an increase of the overall stress for a given applied strain, and therefore contribute to the material's hardening. This tendency is often described as a smaller is stronger size effect. For metallic polycrystalline materials, grain-size strengthening is one the most common mechanisms. This size effect is also known as the Hall-Petch effect [1, 2], which states that the flow stress at constant strain is proportional to the inverse square root of