I was just looking at your arms. If you don't already, eat protein. Protein builds mass and I'm assuming that's what you're looking for.
Weight is very subjective. If it's too easy to lift, then up the weight by 5 pounds and so on. Get your muscles used to the exercise. Plus, form is everything. If you're swinging the dumbell/barbell to get momentum, you're defeating the purpose. I say that because I see guys swinging 50 pound dumbells which does nothing for them. If they were using correct form, they could probably only do half that. What I'm getting at is that if you are using correct form, the number is irrelevant. The object is to fatigue the muscle within 3 sets and if you do that with 20 pounds, then great. You will only advance from there!