I dont like to cook anything thicker then about 4 inches. Remember the thicker the rock the longer it will take to dry and the longer it will need to be held at the peak cooking temp.
As for the rock type it ranges from 300 to 800 depending on the type. some rock will blow up at 350 and some rock will not change at all untill it hits 700 so we would need to know the type of rock you plane to cook.