Yeah, thats true, many modern porsches have 299x24 rear rotors, except turbos etc. Some turbos have same size rotor at the back but thinner. Like 993 turbo, it has front 322x32 and rear 322x28 according to this rough list:
http://www.rennsportsystems.com/2c.html
There is no 944 rotor sizes, too bad. But some other models have same size brakes, like 964.