A 5 ton?? stand alone system?? In a normal home that would be good for roughly 3000 - 3500sqft. You are looking at 2,000 cubic feet of air per minute from that unit. The *MINIMUM* duct size coming for 5 tons is 24" x 24"(square, not round)...do the cross section math. You will need to have 2 very large dampers...one for RA, the other for Supply. Reguardless of heat load, if you try to run it with a single 6 or 8 inch duct, it will be extremely loud and immediately start freezing the coils because of the of the severe restriction. Running 2 zones is the easy part... go get yourself a ductulator and go from there