Hi Mcbain, welcome to THE forum.
The FIA regulation http://www.fia.com/resources/documents/ ... ations.pdf
12.4.1 Complete wheel width must lie between 345mm and 365mm when fitted to the front of the car and between 440mm to 460 mm when fitted to the rear.
But that's the total, absolute width, the rims would be narrower to accomodate tire bulge.