Optimization of heavy materials like steel, in order to create a lighter vehicle, it is a major goal among most automakers, since heavy vehicles simply cannot compete with a lightweight model's fuel economy. Thinking this way, this paper shows a case study where the Size Optimization technique is applied to a front floor reinforcement. The reinforcement is used by two different vehicles, a subcompact and a crossover Sport Utility Vehicle (SUV), increasing the problem complexity. The Size Optimization technique is supported by Finite Element Method (FEM) tools. FEM in Computer Aided Engineering (CAE) is a numerical method for solving engineering problems, and its use can help to optimize prototype utilization and physical testing. As the component geometry was already defined, the Size Optimization becomes the most adequate technique to be used, because it defines ideal component parameters, such as material values, cross-section dimensions and thicknesses, without changing its shape [1]. The Size Optimization methodology is a procedure in which certain parameters (Design Variables) need to be determined to achieve targeted performance (Objective Function) under given Design Constraints. For this instance, Design Variables are: thickness and material's mechanical properties; the Objective Function is to minimize the component's mass; and the Design Constraints are the structural performances in Side Impact Crash, Durability and Equivalent Stiffness tests.