I don't think there is any relationship between head weight and length
of stroke. The avaerage that I have heard of is in the 6-9 inch range.
Typically, the homemade "helve" hammers are setup to be driven by an
electric motor with an off center disc. The stroke length is the
relationship between the distance from the center of the motor shaft
to the center of the off-center rod along with the length from the
pivot arm center to the rod connection AND the distance from the pivot
arm center to the hammer head. All three of these distances can effect
the stroke as well as the horsepower needed to drive the hammer. Now,
once the hammer is assembled, there is some tweeking necessary to
balance the system and keep it from walking around the shop. The
biggest concern that I have read about is making the entire machine
heavy enough to stay put.
If you run a Google search for "baby helve hammer", you should be able
to find 2 JPG files that are hand-drawn sketches by Jr. Strasil that
really does a good job of showing you how to build one.
Hope this helps,