Figured I could maybe give back a little to the people here on PA with these plans.
A couple things you can adjust for a personal fit:
1) The legs. Find a bench or chair that is a good height for you and make the bench that height.
2) The vertical platform support. Once the seat is made, legs attached, and platform cut out, sit on the bench at experiment with different platform angles/heights to find what is most comfortable for you.
3) The foot plate. Since the arm is offset to the left side of the bench, the footplate needs to extend to the right side more so that both feet can reach it comfortably. The drawing is wrong in that the foot plate width dimensions need to be wider to accommodate this change.
One thing I would change if I were to make another shaving horse is to have two arms, one on each side of the bench. So instead of cutting the slot in the platform and the seat for the movement of the arm, I would just attach one on each side of the bench. This would also give equal tension when pressing down with your feet and max holding power. The only disadvantage would be that you have to insert the piece of wood your working on from the front/top.
I just use a wooden dowel for the arm pivot. This can be removed when needing to change the height of the arm to accept bigger logs.
PM me with any questions.