To realize future ultra-broadband ubiquitous, resilient 6G/7G communication networks, the seamless, transparent linkage between optical and wireless networks is required . We have studied the so-called photonic double-mixing functionality of transistors as a candidate of an efficient carrier frequency down-converter . Its down-conversion process consists of photomixing of an optical carrier and subcarrier signals, generating the difference-frequency beat-note of the MMW/THz data signal, and RF mixing of the beat-note MMW/THz data and LO signals, generating an IF data signal. Recently, we have developed a MMW/THz carrier frequency down-converter based on an InGaAs-channel high-electron-mobility transistor (HEMT) with the uni-traveling-carrier photodiode (UTC-PD) structure (UTC-PD-integrated HEMT, see Fig. 1(a) ), demonstrating the significant enhancement of its double-mixing conversion gain by over 34 dB from that of a standard HEMT . The conversion gain of the UTC-PD-integrated HEMT depends on various material and geometrical parameters through cutoff frequencies of the UTC-PD and HEMT, photoabsorption efficiency of the UTC-PD, etc. with complicated trade-off relations, so that the comprehensive device design principles are to be developed. In this work, we studied the scaling rule of the conversion gain on UTC-PD mesa size, and experimentally revealed the conversion gain increases with decreasing the mesa size up to the diffraction limit.