The optimum size of the cavity accommodating a solute in the reaction field theory of solvation is considered by empirical calibration of the results of electronic structure calculations against experiment. To isolate the long range electrostatic free energy contributions treated by reaction field theory from the many other short range contributions not explicitly considered, computational results are compared to experimental determinations of conformational free energy differences in polar solutes having two or more stable or metastable isomers. When the cavity shape is defined by a solute electronic isodensity contour, it is found that the best overall agreement with experiment is obtained with a cavity size corresponding to the 0.001 a.u. contour.