I just had an installer come out and look at my new construction house, we spent close to an hour trying to figure out where a 10X20 duct will run. I remember when I lived on the east coast every room had a heating return grill as well as a supply grill. Is this no longer a "good" thing to do or is it just "easier" to run one big duct and give up a bunch of space? I figure if 1 10X20 is required than at least 4 2.5X5 ducts would be good but 8 2.5X5's split between 2 floors would be more than adequate.

A single 10X20 is not very much return air. You better think that over and while you are at it read the item below before mistakes are made.

It seems like a large footprint to loose from any area, and I think my question would have been better worded: Is it OK to take the single size duct and divide it up equally among several rooms to provide return air? Another part of my problem is the contractor in question is a close relative, but the checklist is very helpful and I will use it with the next person I call ( who is highly recommended by a builder friend of mine).

First off you do not mention the sq ft area of the building you have

Second you fail to mention the equipment size you have.

10X20 return air is small considering you loose 25% as soon as you install a grill on the end of it. Yep grills help to loose 25% of the air flow.

It would always be preferred to have return airs in every room EXCEPT the bathrooms and kitchens. I also do not like to take return airs out of the laundry room areas.

You can run a one piece return air BUT then you have to cut a opening from each room into the hall way OR cut the bottoms of the doors off a bit to allow the air to flow out of the rooms into the hall area where the single return air is located.

Properly sizing return and supply duct is a science not a guess your best situation. Most if not all poor performing HVAC systems are caused by bad choices when it comes to both the supply and return the duct systems.