A computer-implemented method and system automatically adjusts the size of a rigid body model. The method and system construct a two-dimensional model or a three-dimensional model, where the model has one or more rigid bodies. The rigid bodies are converted into geometric primitives that represent a respective rigid body and enable the respective rigid body to resize. One or more of the primitives are constrained to one another. A solver process changes a size of at least one geometric primitive and a rigid body simulation process uses the resized primitive(s) as input.