Terahertz waves bring together the assets of microwave and infrared waves and open up new possibilities in structure analysis and material characterization. In order to optimize a J-band material characterization bench (220-330 GHz), this work proposes a novel simulation tool based on a Gaussian beam formalism enabling the modeling and design of large size quasi-optical components. Numerical results showing the applicability of this framework in the modeling of a J-band lens antenna are provided and validated through measurements performed with a robotic bench.