Electron density of the H ii regions detected in our survey versus the corresponding linear diameter. The points denoting the data (listed in Table 5) are overlaid on an image whose intensity (color scale) gives the number of H ii regions expected in each pixel on the basis of the model described in Appendix A. The solid lines correspond to ZAMS stars of spectral types B3, B1, B0, O7, and O4. The cross indicates the 20% error in the electron density and the 10% error in the source size (see Sect. 5.1).