Gary,
Don’t know if this will help you any as I built mine about 20 years ago so the
decisions making on the build is rather vague. However I took many dimensions
off of one I saw in an antique shop. The main frame is six feet long. The
clamping arm is four feet long.  It really does not need to be that long but way
too much work to shorten it now. That leaves 24 inches for the seat. The pivot
for the foot bar is about ten inches forward of the front end of the seat area.
I estimate that this set up can accomodate a range of 25 to 40 inch legs.

Bill