I'm contemplating a power hammer build and have an idea for one that will occupy a small space. My question or thought is, how big of a hammer is minimal for a unit that relies on gravity? What weight would one need to equal the power of a hand held hammer beating on an anvil?  The addition of some springs would increase the power but by itself. for a hobby type set up, to ease the torcher on ones arm, what would be minimum?

The old power hammers usually start at about 25 lbs for the ram.  I have a 50 lb Little Giant and I think that is minimum at times.  You can always ease up on the power for lighter blows, but at a certain point, it won't hit any harder than the ram weight can produce.  I like the 50 lb starting point.  Good luck.  

I built mine at #50 and like SoCal says i ease up for lighter blows mine tucks in nice in the corner its got a 5'base and 6' i beam for my truck spring to pivot on .