Antenna miniaturization is a necessary technique to decrease an antenna size for providing a compact transceiver system. The optical approach technique adoption in the Terahertz (THz) spectrum, such as using a lens, requires a very small wavelength compared with the optical component dimension to avoid large diffraction effects and fulfill the geometrical optics approach. The impact is a large device dimension obtained after combining an antenna with an optical lens. This paper investigates radiation performances of a size reduced THz lens antenna fed with a planar crossed bowtie antenna affected by applying optimum matching layers. Simulation results by using CST Microwave Studio show that decreasing gain, radiation efficiency, and beamwidth occurs significantly when the reduction is coming to the main beam radiation. The matching also has a contribution to improve those parameters due to its characteristic to reduce wave reflections on the boundary surface. The lens size reduction offers an advantage of more suppression on side lobes. These results can be considerations to design a planar array configuration of THz waves sensors based on the quasi-optical antennas.