Yes, I'm Covering the Existing Pillow Case
I'm covering a couch pillow that measures 18 inches sorta square. It's honestly more like 17 x 18.5, but it's close enough to square that I decided to make a square 20-inch pillow cover for it.
I decided to make my cushion covers extra big so I could slip the pillow directly into it without taking off the blue case that's already in place. These pillows have raw fiberfill inside so this saves me a big step and possibly a big mess.