the same way you scale a wheel 🙂 if you make it bigger in 3dmax, it will be bigger in game too. coordinates is not that easy, you will have to simply adjust your cfg to match the link between main bumper so -;
front bumper(main/01) is the "parent" part, any addon part has coordinates that either are exactly the same or a little different, say if a bumper is longer by 2" it will have somehing like 0 0 2 different from the parent.
so ideally, create new mesh, new cfg, new java etc.. duplicate front bumper 1 for e.g., now import your new mesh, scale it, adjust the position over cfg and then do the loading order, update java with the name and that's it.