Carry a tape measure to the store with you when you are shopping. Measure the length, height and width of the sofa you are considering buying.
Take the measurements home and use the tape measure to map out where you want to place the sofa. Have another person come along behind you to put down strips of masking or painter's tape. This gives you an outline of the sofa on the floor area. You can also use sheets of newspaper for this.
Walk around the room with the tape or newspaper in place. Ensure that the placement of the sofa does not make the room appear cramped or cause people to have to walk awkwardly through the area.