There are a lot of things to keep in mind when selecting the right cookie sheet for your cookies. First, there are a lot of cookie/baking sheets to avoid: If your sheet has high sides, that can affect your cookies' ability to properly rise. Dark sheets can absorb too much heat, making your cookies burn or bake too quickly. Sheets that are super shiny can reflect heat and prevent cookies from browning evenly. Thin cookie sheets can warp in the oven, making things cook unevenly or run together. Insulated cookie sheets can make cookies spread too quickly, resulting in cookies with thin, crispy edges, or they could prevent your cookies from browning altogether.  Cooking/Baking sheets to choose: We recommend a heavy-gauge, dull (not shiny) aluminum cookie sheet that is at least 15- by 12-inches (38- by 30-cm). Your cookies need elbow room. That space allows heat to circulate around the dough, cooking them evenly, and the weight of the pan pretty much guarantees that it'll brown your baked goods to perfection, without warping.