Emission size (average radius of the best-fit elliptical Gaussian) of the molecular lines listed in Table 2 as a function of their upper-level energy. Only the spatially resolved, unblended lines for which a good fit could be achieved are represented. Black symbols show the molecules for which only one emission line is observed and/or resolved in setup S2, while colored symbols show molecules for which several lines are detected and resolved. The error bars are the largest error bar (minor or major axis) produced by the fit with an elliptical Gaussian model. The pink line shows the best power-law fit to the data, as described in the text. All COM lines that are resolved emit in a region of radius ≲100 AU.