For all sizes except for 24x24 and 16x24, select a pillow cover based on the size insert you are using.
16x16, use a 16x16 down insert
18x18, use an 18x18 down insert
20x20, use a 20x20 down insert
For 16x24, use a 16x26 down insert
for 24x24, use a 26x26 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. Lumbar pillows pictured are sewn at 16x24 and stuffed with a 16x26 insert.