I'm remodeling my bsmt into an apt, There are 4 heat ducts but no cold air return for the bsmt (there are 2 on the first floor), Do I need to add one for the bsmt (approx 1200 sqf). If how large. Thanks

I always use what is called a manual D.

Understanding Manual D - Air Flow and Duct Design In The Real World - ACCA Store

I am sure there is a free program for this so look on Google for hvac manual D