What size bungee

I am getting ready to place an order from ACS for parts to build the landing gear for the ultra piet. I plan to do the jenny style with the bungee cords. The plans dont specify a size only that i need 2 3ft lengths i see ACS has 4 6 8 and 19 inch diameter sizes. Which size should i get?

 

I believe it will be a bit of trial and error to get the answer to your question. Do you have a empty weight of the aircraft? I guess I would start with the 6 or 8 and see what the stance of the plane is on the gear. I suppose you can add or remove bungee rings as needed ??