This might help for webbing length. When a customer wants a vintage cotton harness I ask them to make these measurements. By doubling A, B and adding C and D (E might not apply if you don't want a sternum strap) and then adding a few inches for slots, etc., you can get somewhat of the amount of webbing needed. Obviously I have made hundreds of the cotton ones so the amount of webbing for the quick release on the D rings is another known quantity, but you can get an idea of how much you might want to purchase. Remember you can always cut off extra, but adding to existing amount after cut is difficult at least, maybe impossible.
Measurements to take for vintage style cotton harness:
A. Top mark on tank to breast
B. Bottom mark on tank to breast
C. Around waist at location of bottom mark on tank
D. Bottom mark to belly button (between legs/crouch)
E. (Not shown on diagram) Breast nipple to breast nipple