Walk around the room where you want to add the sectional sofa. Use a piece of tape or a sticky note to mark the minimum space you need around the furniture. Place the sticky notes or tape on the wall if you plan to put the sectional against the wall. Put the tape or sticky notes on the floor if the sectional is to go in the middle of the room.
Measure the room from the corner where you plan to put the sectional to the spots you marked on the wall. Measure both the long side and short side of the room. This is the maximum length and width of the sectional to buy for the room. If the sectional is to be placed in an open area of the room, measure the distance between the sticky notes or tape you place on the floor in Step 1 to get the maximum dimensions of the sofa you should buy.
Visit your local furniture store and measure the sectional sofas you are considering. Choose one that comes in at or under the dimensions you identified in Step 2.