I was wondering if there is such a thing as a thin duct? I am hoping to finnish my basement that has a low ceiling and the issue is that there is a full size duct running the length of the room thus causing the occasional head bump but more importantly I would like to close the exposed ceiling with drywall. Can anyone help me on suggestions?

Good morning, I am in the process of remodeling my basement, and I am wondering if it matters what size return vent I use in the room? I live in a townhouse, and the basement is going to be one main room plus a 1/2 bath. The return is going to be in the main room. A friend of mine cut in the new feeds (2) to the room, and bought the return vent for me. The vent he bought is roughly 18" x 18". That vent is larger than any return in the rest of my home...I just want to make sure I'm not going to inhibit the return in the rest of the home. Any input would be appreciated.