In order to study the scale effects and formation mechanism of gas emission from coal particles, we conducted gas desorption and diffusion experiments, investigated the variation of particle size and its effect on gas flow, analyzed and discussed the control mechanism and gas diffusion model of coal. Based on the above, we found that the bidisperse diffusion model was suitable to study the size effects of gas emission and further established a simplified numerical method of bidisperse diffusion model and compared it with the experiment results. The results showed that the accumulated gas emission volume and the initial gas emission speed have scale characteristics. The scale effects of gas emission from coal are determined by the multi-scale pore structure of coal. The diffusion coefficient and path of coal particle determined the gas flow process. Through the gas desorption and diffusion experiments, we can find a way to determine the critical diffusion parameters. On the basis of experiments and theoretical analyses performed, the processes of gas emission from coal particles are studied and their mathematical models are put forward. These models can describe the experimental phenomena fairly.