Our material specs are proprietary to AP Racing, but we do use cast iron for car applications and stainless steel for our motorcycle range. 
A lot of teams manufacture their own discs therefore we can't say if there is a “Standard” size. Ø248mm is a typical size AP Racing has supplied into the SAE.
AP Racing does not make any discs suitable to run with CP4226 and CP4227 calipers. These calipers generally run with standard rear discs supplied with the motorcycle.
We often get many questions asked about drilled and grooved discs so below are listed the pros and cons of using a drilled versus grooved disc.

Grooved discs.

1. grooves clean the pad surface and increase the brake faces (the leading edges of the grooves) so in theory the more grooves the more faces.

2. grooved discs can be used with any pad material as the disc is more durable than a drilled version.

3. discs life is longer than a drilled equivalent.

Disadvantages.
Disc is heavier than a drilled equivalent.

Drilled discs.
1. Lighter weight than a grooved equivalent
2. Good brake bite due to:

A - forcing pad material into the holes
B - increased braking surface (many leading edges in theory)
3. allowing gases formed during the braking cycle to escape from the disc braking face.

Disadvantages.
Discs can be prone to cracking unless care taken when bedding in.
The use of hard race pads (high friction level) can cause cracking giving the disc a limited life.
You will find today that most forms of car motor sport will use grooved discs.