Hi Barbara, is this just to be a toss cushion?, they come in a variety of sizes, for a couch that length you may want to go with the 13" square cushions, they are a great fit and not too small, you could go to a 14" if you want as well, either will look great!
Anything from 12 to 16 inches.
A variety of sizes looks fabulous!
Hi Barbara: The size doesn't matter as much as the design. Of course you don't want a humongous pillow or a tiny one :) I'd get some that would go from 1/2 way to 3/4 of the way from the seat to the top of the back of the couch. A couple on each end would be nice. One smaller than the other. Good luck
Hi Barbara. I think a big sofa should have a few big pillows. They give a look of comfy and cozy. But you could also work in smaller pillows, especially if they match your decor. Good luck Barbara. Hope all is well with you.
It depends on what pillows you already have on your couch that are the same fabric as the couch and what you will be keeping or taking off. Do you have any pics? For instance, my couch has 2 large pillows that are in the same fabric as the couch itself. I just removed them and found a couple of colorful throw pillows to use on it instead. You don't want to overdo it with all the throw pillows that you can't even sit down though. That is why I removed the large throw pillows that had no interest and used my own. But on a couch, any variety of sizes will work. Just try to keep colors similar.
I would buy at least five - maybe seven cushions about 17" square, but there is no reason not to use different size/shape . Start by buying 2 cushion you love. Then add to them. Some people only like 2 or 3 cushions, while others love lots. There is no right or wrong, so go ahead and enjoy the choice.......
Use different sizes and textures. One size or one color or fabric looks boring.
I would do maybe 2 pillows on each side and a lumbar one in the middle. This is helpful: https://www.youtube.com/watch?v=i5N6ajVBbsM
No matter what size we have, they all seem to get stacked on the floor when we have company and need seating space.
Here's a post that might help - https://ariannabelle.com/pages/pillow-size-guide-for-sofas