Sizes of various molecules (expressed as equivalent number of carbon atoms N_C) as a function of their rotational constants B for experimentally measured or theoretically calculated values from the literature (see main text for details). The lines are linear models fitted to these values for individual molecular families and extrapolated to larger sizes. We used the solid lines to determine DIB carrier sizes for linear and spherical species; the dashed lines for PAHs are for illustrative purposes only. The shaded areas represent our results for the λλ6379 (green), 5797 (red) and 6614 (blue) DIBs assuming either a spherical geometry (top) or linear species (bottom). The width of these shaded areas indicate our nominal 1-σ confidence intervals for the derived B values (also indicated at the top and bottom of the figure, with the nominal value indicated by the ◊ symbols) while the height of these shaded areas represents the corresponding confidence interval for the derived N_C values. We note that the B-value for the λ6614 DIB is independent of the assumed geometry.