Dielectric plate gap structures have been extensively applied in micro-discharge units and high-power microwave devices. Therefore, investigating the microwave gas breakdown characteristics in such gaps is crucial for engineering applications. In this paper, a self-consistent particle-in-cell-Monte Carlo collision model is adopted to numerically simulate the microwave argon breakdown process in parallel dielectric plate gaps, and the electron displacement amplitude is derived based on electron kinetic theory. Simulation results indicate that when the electron displacement amplitude is comparable to the gap width under low gas pressures, the enhancement of the microwave electric field simultaneously elevates both the ionization rate and the deposition loss rate on dielectric surfaces. Driven by the competition between the two effects, the net electron growth rate exhibits a trend of first increasing and then decreasing with increasing microwave electric field. In contrast, when the gap width is much larger than the electron displacement amplitude, ionization plays a dominant role in the evolution of electron populations. In this case, the net electron growth rate increases monotonically with the microwave electric field, and both the growth rate and the mean electron energy are significantly higher than those in the small-gap scenario. Under high gas pressures, the electron displacement amplitude is far smaller than the gap widths ranging from 0.5 mm to 5 mm considered in this study, and the electron deposition loss effect is greatly weakened, resulting in a negligible influence of gap width on the net electron growth rate. The simulated breakdown electric field under low pressure decreases with increasing gap width, and this variation trend is consistent with experimental results.