Forced hot air on radiant floor return

mattyt1984

I have radiant floor heat on the 1st floor of my home.  Below I have a basement just finished approx 20 x 20 with 10 foot ceilings.   I have installed a 100,000 btu heat exchanger on the return side of floor heat.  I want to heat the basement it always stays 55 degrees with no heat in the coldest days of winter.  I am going to install a drop ceiling at 9 feet.  I have the heat exchanger in the floor joist and want to add duct work to go thru blow out and return in the ceiling.  My question is what size duct and how many returns/outlets?  My return temp is about 140