Can you sew over a zipper?
When sewing across the zipper, nylon zippers and plastic zippers sew very easily — just use the handwheel and go slowly and you’ll be fine. Be aware, though, that sewing across a nylon or plastic zipper will almost always damage that part of the zipper, so there’s no going back! … Remember that you can pin zipper tape!
What size zipper do I need for a 20 inch pillow?
For example, I use a 18” zipper for a 20” pillow insert and a 22” zipper for a 24” pillow insert, etc. and I always sew my covers 1 inch smaller than the insert (the seams for a 20” pillow insert are actually 19” for the pillow cover) so a zipper that is two inches smaller than the size of the insert works best.