I guess we could either use the same scaling as in the demo, or use the scale of the game engine. Obviously, in computer gaming everything is a construct, so scale only refers the size of objects relative to each other. The player otherwise won't know the difference.
I don't even know what the "scale" was in Redline. What I did to scale my models was compare the models in-game to the crash boxes, which were done in real scale. Probably the units of the crash boxes used by the game engine would be the simplest thing to base the model scale and positioning on, and hopefully the game engine is already designed to make this clear. In addition, if models are one-piece, they can be rescaled as necessary. The body should clearly be one model except for the hood, the wheels should be modelled completely separately since in the SR universe they're distinct from the cars anyway, along with the exhausts, and the engine, transmission, and differential will likely only be relevant in the garage view.
If I have a vote in this, my position is that we should choose whatever scale the engine is designed for to make development as easy as possible.