chances are they are 30mm of some sort because 26mm were only available on non-turbo 300zx's until june of 1990.

let me find asad's webpage if i can. but if u want to tell, measure the width of the groove inside the caliper where the rotor goes in. the 26mm one should measure about 28-30mm (to allow for pads and such) and the 30mm caliper measures out to 32-34mm.

if u have the 30mm it could be an aluminum or iron based caliper. one or the other has "inserts" around where the mounting bolt holes go..i think it is the aluminum ones.