If I could get the .ini setting to work, I think it automatically computes the size values for all new models, which could be hundreds of models.
This matters if you're playing multiplayer mode. In singleplayer, the model.siz doesn't give you an advantage or a disadvantage. If I overwrite the old FDN with a new one that is 10% larger, I get to attack it with overloaded weapons at range 9.2k from my opponent's perspective. And of course, he cannot retaliate with his OL Photons.
This is why model.siz should always be checked in online play. It forces all models with the same name (FDN, FCA, FDD, etc.) to be the same size, or it notifies all players of the model.siz difference.