2) DO consider changing the shape and color of the pillows from that of your base sofa cushion and fabric color. Have many pillows that came with your sofa? Add another fabric to the face of some, and add welting, trim and other details for interest.
4) Square pillows work for many applications. For example, 16" squares are good corner pillows for 6-7' sofas and 18" squares work for 8' or larger sofas. 26" x 26" is a good size for floor pillows and back pillows for queen and king sized beds.
5) Try neck roll pillows. These are long and round - generally 8" in diameter and 16" long for lounge chairs. Bolsters in larger scale 10"in diameter and 22" wide, are great for beds. Scale the width for the chair.
6) Kidney pillows dress up large dining chairs and the center of sofas. The chair pillow should sit just inside the width of the seat and be as high as needed for comfort when sitting in the furniture. This lounge chair has a back pillow with a kidney pillow in front for a very petite client.
7) Add detail with contrast welting, a band with a second fabric, and appliques. How about corner or center buttons?
One decorative pillow has more impact than 3 plain ones.
9) Experiment with mixing round and square shapes together. On large sofas I like to use square corner pillows with a smaller round in front. If the sofa is really long, a large bolster in the center is nice and also serves as a back support.