What size cooling rack should I buy?

Match your rack to your most used baking sheet. A half sheet rack (roughly 17" x 12") fits standard half sheet pans and is the most versatile size. Quarter sheet racks are good for small batches and toaster ovens. If you bake full batches of cookies regularly, buy two half sheet racks. For cakes and pies specifically, a round rack sized to your cake pans is useful.