The temporal behavior of the laser-induced incandescence (LII) signal is often used for soot-particle sizing, which is possible because the cooling behavior of a laser-heated particle is dependent on the particle size. The heat- and mass-transfer model describing the temporal LII-signal behavior has in this work been extended to include the influence of the primary particle-size distribution and the spatial distribution of laser energy. When evaluating primary particle size, a monodisperse size distribution is often assumed, although it is well known that a polydisperse distribution is a better description of the real situation. In this work the impact of this assumption is investigated for Gaussian and lognormal size distributions of different widths, and the result is a significant bias towards larger particle sizes because of the higher influence of larger particles on the LII signal. Moreover, the dependence of the LII signal on the laser fluence is studied for different spatial distributions of the laser energy. The top-hat, Gaussian sheet and Gaussian beam distributions were tested and it is established that the LII signal is strongly dependent on the choice of distribution. However, in this case the influence of particle size is minor.