  A brief narrative on how the effective radius and isophotal diameters were
accepted as galaxy size measures is presented. Evidence suggests that these
parameters were defined only based on observational premises, independent of
any astrophysical theories. An alternative, new physically motivated size
definition based on the expected gas density threshold required for star
formation in galaxies is discussed. The intrinsic scatter of the size-stellar
mass relation using the new size measure is 0.06 dex, three times smaller than
that of the relation with the effective radius as size. The new physically
motivated size measure can be adopted in upcoming deep, wide imaging surveys.
