There no rules for the dimensions like the 1/5th scale categories. But the dimensions are shared between models so if you do a body differents chassis could receive it.
The best choice is the Porsche, there are a lot fo them but you have already made the Rally G. So the second choice will be the R21, it's the first 1/4 from Yankee.
The dimension of the Renault R21 Quadra are:
Front and Rear width : 400mm wheel to wheel.
For the followings car the front and rear are the same as the R21: 400mm.
Wheelbases for the following cars are: 640mm.
Mercedes 190E EVO2 DTM 1992
Mercedes C-Class DTM 1994
BMW E30 DTM
Alfa 155 GTA
Opel Calibra DTM
Last thing is the size of the wheels 150mm or 160mm outside diameter and 70mm width.
After there are the wide bodies like the 905 LM, Jaguar LM, Renault Spider and the Venturi LM.