Finished Basement - Cold air return

Hi,



I have a 750 sq ft finished basement that has 3 6" round diffusers on the ceiling for the entire area. The basement is completely finished and insulated. I am looking to add a cold air return in the basement and have a 24" stud cavity that I could use to run one by the floor. Is lower better or should I do it on the ceiling as well.



If I do use the stud cavity and run it down to the floor, how big of a hole and grill should I install?



Thanks for the help.



Mike