Size-color diagram for the population of radiative-transfer models for the four different luminosity cases (11 L_⊙, 21 L_⊙, 60 L_⊙, and 200 L_⊙). The vertical lines indicate the range of sizes that can be obtained for the individual models. The 5% smallest and 5% largest mid-infrared sizes are shown in transparent colors, giving an idea of the typical (90%) range of model sizes. In black, the models without a gap are shown. The other colors show models with a gap between the halo and the disk components (for an increasing disk inner radius R_in).