What size cookie sheet should I buy?

The half sheet pan (18" x 13") is the standard and fits in virtually every home oven with room for air circulation. This is the size to buy if you're only getting one. Quarter sheet pans (13" x 9") are handy for small batches, toaster ovens, and meal prep. Full sheet pans (26" x 18") don't fit in home ovens they're for commercial use only. Always measure your oven interior before buying to ensure 1 2 inches of clearance on all sides.