Here is an old set of plans I made for myself a long time ago. These are meant for dimensional lumber. This plan is for a short horse. If you are tall or don’t mind the extra length to transport, an extra 6 inches is a good idea. You can see from the other horses that this more of a guideline than an actual recipe.
The hard to figure part is the length of the arm, the head, leg height, and other parts specific to your body and what you generally make. I’ll measure more and put up something better when I can.
Preindustrial Craftsmanship 