The best pillow arrangement for your Livingroom sofa will all depend on how large your sofa is.
If you have a settee try an 18x18 and a 12x20 pillow at one end of the sofa.
If you have a sectional try three pillows at the corner unit in a range of sizes: 22"x22", 20"x20", and 18"x18". At each end of the section try one 22"x22" and one 20"x20".
If you have a sofa try one 22"x22" and one 20"x20" at each end of your sofa.
If you have a club chair, try a 12"x20" lumbar pillow.