Can you predict how big your triceps can get? In my experience, yes. There are two predictor lifts for this
You can do as many triceps pushdowns and cable extensions as you want, you will never have big triceps unless you can overhead press and close grip bench press impressive weights.
If you were to ask me what the best two way to determine the size of one triceps without seeing them, it would boil down to this:
1. how they can bench press for 6 reps in relation to their bodyweight.
2. how they can overhead presses for 6 reps in relation to their bodyweight.
Look at the triceps of my colleague and friend Josh Bryant, he was the youngest man to bench press 600 lbs raw.
Regarding the overhead pressing strength, two of the largest triceps in history belong to weightlifters Phillip Grippaldi and Victor Sots.
Why is overhead pressing 6 R.M. so important in developing large triceps. Because overhead presses have been show to be the best activators of the long-head of the triceps. You will never have the biggest triceps possible unless you maximize your overhead pressing strength.
BONUS # 1: How do you know if your need to do more overhead work. Simple, your best 6 reps in the seated press behind neck should be about 2/3 of your close grip bench. So if you can do 100 kg for 6 reps in the close grip bench presses, you seated press behind for 6 reps should be 66 kg. Not convinced?
BONUS # 2: Are you terrible at overhead presses?
Here is a routine for you: Improving Overhead Pressing and Chin-Up Strength: A Twelve Week Cycle
Enjoy the new growth,
Coach Charles Poliquin