Fundamentally important are realizable experimental procedures to measure some aspect of size, whatever that may mean in the quantum realm of atoms and nuclei. Foremost, the nucleus can be modeled as a sphere of positive charge for the interpretation of electron scattering experiments: the electrons "see" a range of cross-sections, for which a mean can be taken. The qualification of "rms" (root mean square) arises because it is the nuclear cross-section, proportional to the square of the radius, which is determining for electron scattering.