Please review the pillow sizing guide which will help you choose an appropriate size for your furniture. Keep in mind that the pillow covers dimensions are of the flat 2-dimensional cover. Once you fill it with an insert, some of the height and width of the flat cover will wrap around the curve of the 3-dimensional insert. This means that visually speaking, the 3-dimensional stuffed pillow will not translate inch for inch from the size of the 2-dimensional cover, but will look smaller. Therefore, err on the side of ordering a larger size if you are on the fence between two sizes.Below are links to photos of how each pillow insert looks on an average 2 seat or 3 seat sofa to help you visualize the relative sizes:12x18 inches12x24 inches14x20 inches16x16 inches16x26 inches18x18 inches20x20 inches22x22 inches24x24 inches26x26 inches