For all sizes except for 24x24 and 16x24, select a pillow cover based on the size insert you are using.
20x20, use a 20x20 down insert
20x20 covers are sewn at 19x19. Otherwise, the fabric will hang on the cover and look sloppy. This is to ensure the best look- full overstuffed and cozy as pictured in all my listings. All covers pictured in square sizes are sewn this way and stuffed with a 20x20 insert.