How do I know what size to build the heat distribution box? It is located in my crawl space and has some leaks from rust/rot. I have rerouted and added 2 extra vents. Was told if the box is too small it will choke out the furnace, if too big I will not get any pressure through the ducts to the vents. Please can someone help or tell me a formula to use.

You may want to clarify your question.
Assuming the heat distribution box was matched to the current furnace (or was working fine before additions to the distribution ducts) I expect it functions to store a common pressure supply for each of the vent branch ducts that were designed to have a balanced pressure/flow gradient drop to the areas served by the vents. Rerouting and added vents could change the balance of the gradient drop depending on where the additions were installed and how severe the rerouting altered the flow path (either more or less restrictive).
If the extra vents were added with new branches and the branches were added:
Symmetrically at the distribution box,
With the same size and length of ducts,
With relatively matching routing and vent spacing,
With equivalent areas served
; virtually identical to original branches, I would expect the common pressure supply to remain acceptable.
If the vents were "tapped" into existing branches I would expect a new air flow distribution calculation (try gooogling it) would be in order.
I'm not sure if your concerns are with the size of the distribution box or the sizing of ducts (or maybe vent openings as a work-around).
I understand your need to alleviate the leaks in the distribution box.
I am only answering this as your sounding board.