For developing innovative filter media, simulation of filtration performance on virtual 3D fibrous microstructures is a key enabling technology. Typical steps on the way to a virtual clone of a real filter medium are briefly described, but the focus is on image processing. Due to the limitations imposed by available computational power and time, a small simulation domain is desirable. Therefore, a numerical study is carried out to determine the minimal representative domain size for a given air filter medium. It turns out that the percolation path, i.e., the path of the largest penetrating particle, correlates quite well with dust holding capacity and might be a better measure than pressure drop for a representative domain size. As an outlook and an example, it is demonstrated that the performance achieved by a virtual clone agrees well with the one of the original real media based on which the clone was modeled.