<p>A direct numerical simulation framework was established to investigate droplet collision in a gas medium in this study. This problem is a multiscale problem owing to the intervening gas film flow that spans from a continuous flow regime to free molecular flow regime. A dual-volume of fluid numerical method was employed, incorporating rarefied gas effects, van der Waals forces, and an adaptive mesh refinement algorithm with an improved gas film thickness calculation method. The emphasis of the investigation is the size ratio effects on the gas film drainage and the transition of collision outcomes. Besides the well-known fact that coalescence is promoted by increasing the size ratio, a trend of suppressed coalescence was observed as the size ratio increased to a certain value. As the size ratio increases, gas film length increases moderately while drainage velocity rises more rapidly, reducing drainage time and promoting coalescence. With further increases in size ratio, the gas film length continues to increase, while the drainage velocity increase becomes more gradual, resulting in a longer drainage time and therefore a suppressed coalescence.</p>