We present an analysis of the dependence of the line shape of four-wave-mixing (FWM) signals on the sample geometry and the excitation conditions based on ZnSe single quantum wells with varying width and barrier composition. We analyze the geometry dependence of the biexciton binding energy and observe a strong influence of reabsorption and of the signal polarization on the spectral structures. The results exhibit excellent agreement with calculations based on a microscopic density-matrix theory considering the biexcitonic continuum for a 2D model.