There is no rule of thumb on the size of a rug to place it under the sofa in the size guide. If your 8×10 rug is under sectional, then you have to use a little larger space covering the rug. You may take a 12-18” space covering rug for your sofa. The height of the furniture leg is also important before selecting the rug size.Contemporary living room with stylish gray fireplace matching the rug. Designed by: BFDO Architects. Living room with hardwood flooring and white walls together with a fireplace and stylish wall design. Designed by: MU Architecture. Stylish living room with skylight featuring a brown couch and a black rug. Choosing The Proper Rug Size For A 75" Sofa: When selecting a rug size for a 75" couch, a 5' x 8' or 6' x 9' rug can work well. What Size Rug Should You Choose For A 91" X 91" Sectional: For a 91" x 91" sectional, a square 8' x 8' or 9' x 9' rug can work well, or a rectangular rug that is at least 8' x 10' or 9' x 12'. .