Left: The H I mass-size relation of H I clumps in AGC 226178 and literature sample of almost dark H I clouds. The size of each clump is computed as $D=2\sqrt{A/\pi }$, where A is the projected area estimated from the geometric mean of the major and minor axes in Table B.1. Dashed lines indicate the critical surface density for H I-to-H₂ transition predicted by different models and physical conditions, as indicated in the legend. Stars indicate five almost dark H I clouds investigated by Cannon et al. (2015). The grey lines mark the critical surface density for the atomic-to-molecular gas transition, as predicted by some models. For a detailed discussion of the physical significance of different thresholds, see Section 3.5. Right: The size versus velocity dispersion distribution of H I clumps. In both panels, H I clumps associated with H II regions are plotted with blue filled squares, whereas the others are plotted with filled grey squares. The solid line in the right panel represents the best-fit size-velocity dispersion relation. The best-fit logarithmic slope (k), intercept (b), and the Pearson’s correlation coefficient and p-value are also indicated in the right panel.