Carrier, modal 58MAC120-20, direct air forced air furnace, seven years old in a 2,700 square foot home seven years old. The basement is 1,200 square feet, open and half finished, has two supply air regesters and no return air vent. What size "opening" is recommended for this area?

You will find that this is an debatable issue. One professional will tell you you should not have them and another will say you should. In your case, were you advised, based on some symptom you're having, that you should add them?

A small 8" X 20" return would be fine...