Discs...
 

What size are the front and rear discs ?

There are Discs and Discs Mate.
 

First the vented discs in the Mark 1 prior to 1981 and the Mark 2 after 1982 are different.
The diameter of the front rotors are the same but have different mounting methods to the hub.
Books says FRONT: CL28V 9.92 inches 252 MM.
REAR 1979-1981 AN14H 10.59 inches 269mm
REAR 1981 onwards CL14H 10.16 inches 258mm

The FRONT are the same diameter but the mounting of the series two rotor is via four symmetrical tabs which allows better airflow incoming over bearings at front and rear. The series one mounts flush/interference fit and the incoming air flows in from the rear. Also I have ground both 79/80 and 81/82 and I think the air passages on the 81/82 are larger vents
The brakes pads for series one and two are different model numbers but will interchange FRONT only.
The REAR brake pads for series one and two are totally different.

The diameter of the rear rotors is the same HOWEVER the early series 1 rotor/disc sits back four inches from the wheel mounting face whereas the series 2 is further outboard only two inches from the wheel face. So the rotors are not interchangeable.
The backing plates of both front and rear are improved on the series 2. Suggest you buy Haynes books on 280ZX which is different to a 280Z.

Thanks for the reply..

I take it they are just single piston brakes ??

Yes single caliper just like front of Toyota 4cyl Camry.