I would find out their opinions on rag quilts before I made them one. I can see where some people might think it looks, well, raggedy.

If you know the couple or their family well enough to have been invited to the wedding, don't you know them or their family well enough to inquire as to their color preferences? And do they like traditional things or are they strictly modern? I would think a bed sized quilt would be fine. If they didn't like it very well they could always use it as a blanket under their comforter. It would help keep them warm. If that thought bothers you maybe you could just make them some pillows for their sofa and a matching lap quilt, of course then you definitely have to ask what their decorating color scheme is or risk their not ever using your gift and probably not sending a thank you.

Every one I have made a rag quilt for loves it, I even made one for my bed. I like my room very cool to sleep and the rag quilt is just perfect on our cold Ohio winter nights.