This post really kind of reminds me of myself. I am 13, 6 foot and 150 pounds. My step dad is 6'3 310 pounds and for general forging i use a 5-6 pound hammer and i use it really well (not a ding on my anvil yet after using it 6 days a week for a year). But my step dad was using my old anvil with my favorite hammer(a big no-no!) and you could just see him struggle and with every hit he either hit the steel half way and put a ding or just plain out missed. So it doesn't matter if you use a 6 pound hammer or a 1 pound hammer, it matters only if you move metal and how efficent you are at doing it.