Half sheet pans are 13" x 18" with a 1" side. A full sheet pan is twice the size @ 18" x 26" - too big for most home ovens. a quarter sheet pan is 13" x9", a handy size to have around. They are plain aluminum and made by dozens of different companies. You can get wire racks to fit in them.
Restaurant supply is the way to go for this generic item.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan