The sign for "couch" starts with the sign for "sit" then uses classifier "c" handshapes
to show the relative size and shape of the couch.
Sample sentence: "What color is your couch?" = "YOUR COUCH, what-COLOR?"
After you've established that you are talking about a couch, you can drop the first
part of the sign. Or, if you are already talking about furnishings in
your living room you generally don't need the first part of the for "couch."
Here is an animation of me just showing the hands sliding apart: Couch