Advice on how to build bigger arms?

In order to aid powerful hitting, you don't need Bigger arms, rather u need Stronger arms. you should focus more on strength training techniques. Do some pull ups, Push ups and dips and along with that you should lift dumbbell or barbells. Try to progressively overload your muscles in each set. Lift maximum possible weight with which you can do7-8 repetations in your working set(last set). This technique is also called HIT(High Intensity Training). You can see changes in your strength within 2 months. Last but not least...Eat well. :)