A quality baking sheet should last you for years—generations, actually. I still have one of my grandmother's baking sheets that is a favorite of mine. I like a half-sheet pan in terms of size, which is 18 by 13 inches and is fairly common. I will also add that I'm not a big fan of nonstick cookie sheets. They are typically darker in color, and cookies bake much quicker than on a standard aluminum pan. I actually prefer a basic aluminum pan. And a step up from this is a double-thick aluminum pan, which can be used for everything from baking cookies to roasting veggies.