Using density functional theory calculations, we propose that the exposed Ga atom in a two-dimensional defective gallium selenide monolayer (V-GaSe) can display a good dinitrogen fixation capacity and an excellent nitrogen reduction reaction (NRR) performance. Our results show that N-2 can be captured by three sp a -hybridized Ga atoms due to the pulling effect. With the enlargement in vacancy size through applying tensile strain, the adsorption of N-2 is strengthened and the electrochemical NRR performance is enhanced. On 8% strained V-GaSe, the estimated onset potential is as low as 0.30 V. Inspired by the concept of "defect-size-dependent" NRR performance, we further design a Janus V-GaInSe2 structure in which the natural size of the cavity is enlarged and the electron density of the active Ga atoms is enriched. It is found that N-2 adsorption is demonstrably enhanced with respect to V-GaSe. On 4% strained V-GaInSe2, the onset potential is calculated to be 0.31 V, which is the same as the 8% strained V-GaSe. Moreover, the produced NH3 can be removed rapidly with a free-energy change of less than 0.52 eV, which is much lower than those of most reported catalysts with low overpotentials. Meanwhile, the side hydrogen evolution reaction is successively suppressed as the strain increases. Our work offers a feasible method that utilizes the size of a defect to tune the NRR performance, adding a new understanding of N-2 fixation and sustainable NH3 production.