I bought the 20-inch size to fit my 20-inch pillow covers, and they fit well. My covers are a sort of linen, with a rustic, unfussy look, so I don't mind that they don't appear overstuffed. If you like a more overstuffed look, I would go with a 22-inch insert for a 20-inch pillow cover. The inserts have a sturdy construction, and they seem to be doing the job well! I'm adding 2 photos so you can see one pillow in the case and one out of the case.