Twin size blankets and comforters typically measure between 66 to 68 inches wide by 90 inches long. This size should also fit a twin XL mattress. To account for depth, you can add the width of your mattress and the height on both sides to determine how wide your blanket or comforter should be. For example, most twin mattresses measure 38 inches wide. If your bed sits 12 inches high, you would need a blanket that is at least 62 inches wide to fit your bed. This quick measurement will ensure the blanket fully covers the mattress and hangs down far enough on each side.