A US quarter-sheet pan is typically about 13 by 9 inches at its outer edges. It suits small batches of cookies, vegetables and other foods, and can organize ingredients during preparation. Its usable interior is smaller than the nominal size.
It has roughly half the footprint of a half-sheet pan. A shallow sheet pan is not interchangeable with a deeper 9-by-13-inch cake pan for batter capacity or baking time.
Reference: Further reading.