Vessels that use a custom .dll (listed under "Module = ... in a config file) can not have many of their parameters edited. DryMass and FuelMass are defined in the .dll. And don't even try editing a .dll by hand...
Vessels that use SC3 have everything defined in the .ini file. You can make changes there.
Vessels that use a plain .cfg file have parameters:
Mass refers to the dry mass, MaxFuel refers to the fuel capacity.