You can use an 8" x 8" pan, and your brownies will be a bit thicker. Use an 11" x 7" pan, and your brownies will be slightly thicker. If all you have is a 9" x 13" pan, you can use that, knowing your brownies will be very thin. You'll have to adjust the baking time any time your pan size differs, so just keep an eye on your brownies.