Too tall, you want to be able to see the outline of the lovely chair.

Is the chair too deep or you just want more interest? Maybe a lumbar pillow?

Too big, too tall. Lumbar would be my choice; have one in a wing chair in my living room now.

Thanks, I agree that lumbar might look better, but the problem is I’d like more support for my mid to upper back in the chair.

Since I can customize the pillow size, maybe a larger rectangle that is as wide as the chair but an inch or two shorter than the back?

That looks better, but I'd still go a tad bit smaller. I'm thinking for center back support it might not need to be as wide as the whole chair back? Maybe more like this proportion-wise (even though some of the backs are taller than yours):

Thanks, I’ll go just a little smaller, now I have to figure out which size pillow form!

the back of the chair is 17” wide and the square pillow is 21”, but when it’s fluffed It’s actually about 16-17” in the middle. I was thinking 15x21 Insert? Or do I get a 17x25 insert then make the cover 15x21?

Lumbar pillows work best on chairs IMO and should help with back support, I guess my question would be why did you buy a chair that was not comfortable .

So a square pillow won't work for you? If you use a rectangle, then I'd go even a little narrower, side to side, if it's going to be taller.

Why not get a few lumbar that are returnable and just see how they feel on your back in that chair.

Patricia - My husband bought me these chairs to help because I couldn’t make up my mind what I wanted. :)

This square is 17.5”......

I actually don’t use these pillows anymore, so I can recover them and not have to buy new inserts for now