I did my due diligence and read through this entire thread before posting. 
I was looking at some pre race pics today and saw lots with exposed brakes and some of the variety in the builds. So with the rotors themselves not having restriction as the calipers do, is the only think limiting rotor size the size of the 13 inch wheel itself or is the rotor size restriction indirectly controlled by the caliper size restriction?
Additionally, what's the combination now carbon rotors or are they carbon ceramic? and the pads are ceramic with copper or something more exotic?
Disc dimensions are controlled. Materials are not.
ARTICLE 11 : BRAKE SYSTEM
11.1 Brake circuits and pressure distribution :
11.1.1 With the exception of a KERS, all cars must be equipped with only one brake system. This system must comprise solely of two separate hydraulic circuits operated by one pedal, one circuit operating on the two front wheels and the other on the two rear wheels. This system must be designed so that if a failure occurs in one circuit the pedal will still operate the brakes in the other.
11.1.2 The brake system must be designed in order that the force exerted on the brake pads within each circuit are the same at all times.
11.1.3 Any powered device which is capable of altering the configuration or affecting the performance of any part of the brake system is forbidden.
11.1.4 Any change to, or modulation of, the brake system whilst the car is moving must be made by the driver's direct physical input, may not be pre-set and must be under his complete control at all times.
11.2 Brake calipers :
11.2.1 All brake calipers must be made from aluminium materials with a modulus of elasticity no greater than 80Gpa.
11.2.2 No more than two attachments may be used to secure each brake caliper to the car.
11.2.3 No more than one caliper, with a maximum of six pistons, is permitted on each wheel.
11.2.4 The section of each caliper piston must be circular.
11.3 Brake discs and pads :
11.3.1 No more than one brake disc is permitted on each wheel.
11.3.2 All discs must have a maximum thickness of 28mm and a maximum outside diameter of 278mm.
11.3.3 No more than two brake pads are permitted on each wheel.
It all goes back to overdependence on wings. If they didn't generate so much downforce, neither could they generate so much braking force. Before Bernie became
Lord High Muckety-Muck of F1, cars had enormous tyres, low noses and tiny wings. The FIA thought reducing tyre width would slow the cars but it just encouraged engineers or optimise aerodynamics.
Go back to über-wide tyres, low noses and smaller wings -- which will restore the dependence on mechanical grip -- and the aluminium calipers will be adequate.
