Big arms mean that you will need bigger biceps and triceps. There are 3 heads to the muscle lateral, long and medial head. You cannot isolate these, but you can target them. Exercises that are done with elbows overhead put more emphasis on the long head of the muscle the largest of the 3, like overhead extension. A straight bar cable press pushdown with a reserve grip, will work the medial head. Lastly to work the lateral head, cable kick backs with your hands facing your thighs. These 3 exercises will help to build bigger triceps and wide arms overall.