Compare cushion covers and inserts as separate purchases
A styled pillow photograph can hide two separate products. Check the cover, insert and fastening before comparing prices or dimensions.

The listing says “pillow,” the photographs show a plump square, and the small print says cover only. It is an ordinary source of confusion. Start by checking exactly what is included, particularly when the low price appears alongside several sizes. A cover and an insert bought separately can be useful, but they should be compared with a complete pillow at their combined cost.
Measure what you already have
If replacing covers, remove one and measure it flat from seam to seam. Check the insert's label as well as its current shape; an older filling can settle into a size that is difficult to estimate by looking. A manufacturer's sizing recommendation is a better starting point than automatically buying an insert a fixed amount larger than the cover. Overfilling can strain seams and make a small zipper difficult to close.
Think about where the pillow will go. A deep sofa may accommodate a large square behind a smaller lumbar shape, while a narrow chair can become unusable under the same arrangement. Test with pillows or folded towels you already own. Sit down rather than only looking from across the room. The useful number is the number that can stay in place while someone uses the seat.
If the sofa already has two usable inserts, begin by trying different covers on those sizes rather than inventing a new arrangement. A larger patterned cover may hide the sofa fabric you wanted to keep visible, while a smaller lumbar can add a color without taking over the seat. Put the trial arrangement on the sofa and use the room for an evening before adding more.
Check whether a patterned cover has a repeat that will vary between individual pieces. Two covers from the same fabric may not show the motif in exactly the same position. That can be part of the appeal, but it matters if the desired look depends on a perfectly mirrored pair.
The insert affects more than appearance. Different fillings vary in firmness, how quickly they flatten and the care they require. Choose according to the label and household needs, including any relevant sensitivities. A crisp-looking cover will not make an unsupportive insert feel firmer, and a dense insert may be heavier than you want on a small occasional chair.
Inspect the cover as a washable object
Check whether trims, embroidery or contrasting fabrics change the care instructions. A removable cover is not necessarily machine washable. If you want to rotate covers regularly, a generously sized opening can be easier to use than a short hidden zipper. Look for photographs of the back rather than assuming the pattern continues on both sides.
Keep one or two covers for each insert rather than accumulating a new filled pillow for every season. Fold the spare covers together by size, so replacing a faded one does not become another measuring exercise. If the color is difficult to judge online, order a single cover first where the return terms allow. Living with it beside the sofa fabric is more informative than comparing six nearly identical thumbnails on a screen.