Criticality type eigenvalues of the one-speed transport equation in a homogeneous slab with anisotropic scattering and Marshak boundary conditions are considered. The connection between the transport equations for a critical and for a time-decaying system is established, and thus the time-dependent equation is reduced to the stationary one. Variation of the size of the time-dependent system with anisotropic scattering is studied numerically. Calculations for different combinations of the scattering parameters and the selected values of the time decay constant using the PN method are reported. Results are discussed and compared with those already obtained using various methods available in the literature.