At least 2 feet wider than the sofa, maybe more. If a reversible chaise it is probably on legs so should go entirely on the rug. If it sits flat to to the floor, the rug should slip under the front 1/3 of the bench part and be entirely under the chaise section. Is there other seating that should be on the rug: chairs, coffee table, sofa table...

The rug should be almost as large as the room, with about 18 inches away from the wall on all sides. Smaller rugs don't do the room any favors and just make it look chopped up.