The spontaneous combustion of coals is a natural phenomenon that causes serious losses in the mining sector as well as environmental problems as a result of greenhouse gases released during spontaneous combustion of coal. In this study, for the first time, the effect of coal reactor size on the spontaneous combustion tendency parameters (crossing point temperature (CPT), average heating rate (AHR), and liability index (IFCC)) of the coal samples having two different ranks (bituminous coal and lignite) have been searched by the use of crossing point method. According to the results obtained from the reactors manufactured in three different sizes (R-1, R-2, and R-3), ignition temperatures of coal samples were 161–172 °C for bituminous coal and 143–175 °C for lignite; average temperature rise was 0.92–1.45 °C min−1 for bituminous coal and 1.47–2.98 °C min−1 for lignite; spontaneous combustion risk index was determined in the range of 5.61–8.88 min−1 for bituminous coal and 8.39–20.79 min−1 for lignite. As the reactor size decreased, spontaneous combustion tendencies of lignite samples increased, and no significant change was observed in bituminous coal samples. It has been found that the effect of reactor size on the spontaneous combustion of coal gives significant results in low-rank coals but has no effect on high-rank coals.