Re: Return Air sizing
A 14" return duct is actually OK, as long as it's not flex duct, or exceptionally long.
Ductwork is usually sized for simple systems using a friction of 0.1" of water per 100 ft. of duct. Based on this, a 6" duct is good for 110 CFM (cubic feet per minute), and a 14" duct is good for 1000 CFM, which is greater than 880 CFM (the capacity of (8) 6" ducts). The air velocity would be 935 feet per minute, which might be a little noisy.
As a check, the output of a 75 MBH furnace at 85% efficiency is 64,000 BTUH. For a 60 degree temperature rise, you would need 64,000 / (60 deg. temp rise x 1.08) = 984 CFM.
I don't know of a simple formula to calculate CFM vs. friction. I use a duct sizing slide rule that an HVAC supplier gave me. HVAC textbooks have charts that give the same results.