It doesn't matter how big in inches they are, as long as they look big compared to the rest of your body. I put on about an inch on my last tren cycle, but I did arms 2-3x a week. I got good results with spider curl for reps.
Well that's the thing. My shoulders are big and my back and legs are big, so my arms are disproportionately small I feel. My shoulders respond very well and grow even if I don't hit them directly, so its hard for me to catch up w arms.