The aim of this work package is to develop validated design rules for combining different size effects. This work will build on the published findings that individual length-scales combine to form a characteristic length that determines the strength in a material. Existing state of the art models addressing the different aspects of this plasticity size effect will be accessed from the literature. Physics-based, size-dependent constitutive property relationships will be developed, for both individual and combined length scales, as a function of temperature. Model predictions will be tested against the experimental data from WP3 to select key aspects and components for a new constitutive relationship. This constitutive law will also be informed by a coupled discrete dislocation dynamics model and then validated by the experimental data from WP3. The algorithm of the selected constitutive law will be implemented in a finite element model (FEM) to simulate experimental results and support experimental property and/or residual stress mapping (by indentation) of surfaces and small volumes of material.