A new method is introduced to analyze the accuracy of phase-Doppler anemometry (PDA) for sizing large particles in two-phase flows. The method is based on Fourier optics theory (FOT) and geometrical optics theory (GOT) to calculate the intensity ratio of refractive and reflective light scattered by a sphere under a slit constraint. To examine the accuracy in calculating the light intensity, the results from the GOT method were compared with those of the direct simulation based on the generalized Lorenz-Mie theory (GLMT). This newly developed FOT/GOT method also predicts the results of phase jump due to the slit constraint. The performance of dual-mode phase-Doppler anemometry can also be simulated by this method.