It's typically recommended to use a pillow insert larger than the pillowcase for a nice plumpy pillow, especially for the older flattened pillows or fiber fill. Sometimes, for 100% Hypoallergenic Polyester Inserts, after filling, pillow will decrease about 10% - 15% in size. The toss pillow covers feature 16"x16". So, for best results, now a tad larger inserts are recommended to buy together with the items to provide full pillow from corner to corner.