Oh boy.
Short answer : there is no easy way (at least, known to me).
Long answer :
create a copy of the car.
Extract car's kn5 into 3Ds or Blender or same, then follow this (for blender only):
- delete all the nulls (keep object transformation for attacheds so nothing moves), as they will most likely be broken in the process
- right click - snap - cursor to world origin
- select ALL meshes at once
- right click - set origin - origin to 3D cursor
- Resize your car
- redo ALL the nulls necessary (remember : always Z forward from the move axis)
- redo ALL the animation necessary (same as above)
- export whole car in FBX (size 0.01, Z forward)
Then, redo the car, car's shaders and animations with ks-Editor.
Change every data's info about size for the correct one (inertia, wheel base, suspension, etc)
You'll probably need to change driver's position in CM, also.
Test the result. There will be bugs. Correct the bugs, re-test until satisfaction.
Then do as above for the 3 other LODs and the collider. The good news is : barely no nulls to recreate this time
Conclusion : If Ilja manage to create an easy "re-size car" fonction in CM, I officially declare him as a god.