Hey Offkey, for 5 bucks I'll tell ya.....
you need to go into your sfc3/assets/models/space folder... there are about 30 space models... starting at space00.mod right on up through... all you have to do is take your model that you want to use and replace the space00.mod, then space01.mod etc...
Bit of advice, test one at a time, as it is fairly random which space model gets pulled up. You can test it in single player campaign mode or single player skirmish mode.
The sizing is a little hard to work out, but not too bad.
Hope this helps!