I am fairly certain that this line:
in SystemTypes.txt referes to the Stellar Size line in PlanetSize.txt, not the name.
Also, adding any planets above the Huge planet entry causes weird effects (something like all the specific planet sizes called in SystemTypes.txt get shifted down by as many new planets as you enter in PlanetSize.txt, so one planet added makes all Huge calls become Large planets instead; it is weird 
), and I recommend you do not do it.