Extensions, meaning anything in which the elbows are up around ear level are essential for max. size because they hit the long head, the largest part of the triceps, better than anything else. Not skulls-with extensions, the weight has to be lowered down below the back of the head, getting a complete stretch.
Choose one, lowering the weight behind the head as far back as possible: lying, seated, decline or incline, free weights or pulley.