Although the use of a dielectric lens has shown the gain improvement, the hemispherical shape gives other considerations, which are an effective function of the whole lens, and occupied size just for one sensor. This research proposes a size reduction technique to reduce lens size. The reduced size consideration is necessary to provide an as small as possible sensor of a planar crossed bowtie feeding antenna at a working frequency of 1 THz that radiates THz wave radiation and then collimated by a hemispherical dielectric silicon lens. The initially considered lens diameter and the extended dielectric thickness are 3000 µm and 1000 µm, respectively. This research conducts those investigations by using CST Microwave Studio. The results show that reduced performances of gain and radiation efficiency. However, the gain decreasing slowly at the beginning of the substrate reduction means that the expected gain remains to obtain. The gain still can be achieved at around 30 dB when the extension is about 1000 µm. This technique offers more suppressed side lobe levels and reduced the volume use of dielectric materials. The research can contribute to farther research to develop a compact THz wave sensor device based on a planar antenna and a quasi-optical dielectric lens.