What you are asking is all part of proper duct design. The size varies depending on length, how much air the room(s) need, what type and how many fittings are involved, and how strong the blower is. In the case of in ground duct insulation value plays a part. Finally the type of duct system will make a difference, it can be radial, perimeter loop or possibly a combination of the 2.
Your HVAC company should be able to explain all this to you, if not you might want to consider finding one that can. Price is always a consideration but the cheaper companies can't afford to take the time to do all the engineering to properly lay out a system. Depending on the layout of the house it may take as long for a proper sizing/layout as the time it takes for the cheap guys to do the entire install.