I finished 2/3 of my basement, ending up with a 26 x 34 finished area. We have always had a fairly comfortable area and insulated the poured walls with 1 1/2 blue board, and carpeted the floor. I did install 2 heat flows off the trunk and never gave much attention to a cold air return. Now the room is finished and comfortable but I want to move the air, and need to determin the size of a cold air return, and weather I can just tie into the coal air return or on the bottom of the duct work near the blower motor housing.

Your basement seems about the same size as mine. When I had the heating guy come over the other day I asked him a similar question. As far as the supply goes, he said I could get away with a sqft:cfm ratio of about 1:1. For the return, he recommended a single 12" diameter vent on the wall that separates my utility room from the rest of the basement. The new return duct would tap into the main return just before it entered the air filter. He said the one large return would work because my basement is all one room.

I am definitely not an expert in this area. I am just passing along the information that was given to me the other day based on what seems like a similar setup to mine. Good luck.

You say he recommended 12"... I was thinking about an 8" return myself, but I am no heating/cooling expert. My furnace is just the other side of the finished wall so just 1 return will be simple work for me.