Originally posted by tjoneslo:
I don't think it's explicit anywhere, but D20 has a scale modifier for the smaller/larger creatures:
Large x1.5
Huge x2
Gargantuaan x4
and so on.
Since Starship/vehicles are in volumes (not linear sizes), I'd suggest mulipliying by the cube of the above linear muliples:
Large: x3.5
Huge: x8
Gargantuan: x64
And so on.