It is widely known that the pore size of a meltblown fiber assembly extensively affects the final applications of its products. We have developed a model for simulating melt-blowing production to investigate the formation mechanism of a fiber assembly. In this study, we calculated the pore size under different production conditions using the model. The predicted results reveal the relationship between the pore size and the production conditions, namely, the air jet pressure, suction pressure, die temperature, polymer flow rate, die to collector distance, and collector speed. The predicted results also verified the experimental trends reported in previous studies. High air jet pressure and die temperature tend to generate smaller pores, while a large polymer flow rate, die to collector distance, and collector movement speed contribute to the production of larger pores in the fiber assembly. In addition, the circularity was predicted in this study to describe the pore shape. The numerical investigation of virtual production is a novel method in which the expected pore size and corresponding production conditions can be easily obtained using a computer with a few keystrokes and mouse clicks.