I googled it and the site I found said 9x12 was 1/4 and 11x15 was 1/2 and I'm not sure about a full.

Someone on CC said 11x15 is 1/3.... someone please tell me the right dimensions..

Tina

below is what i found on the internet one day when i wanted to know the same thing.

There is the 1/4 sheet cake that measures 9 inches by 12 inches. A 1/2 sheet cake measures 11 inches by 15 inches. A full sheet cake measures 18 inches by 24 inches. Notice that the 1/2 sheet cake is neither twice the size of a 1/4 cake or half the size of a full sheet cake. It's actually only 50% larger than a 1/4 cake

A half sheet cake is 12"x18".

1/4 Sheet = 9x13
1/3 Sheet = 11x15
1/2 Sheet = 12x18
Full Sheet = 16x24 (baked in 18x26 Bun pan with bakeable cardboard tray).

Additional full sheet info:
A commercial Bun pan is 18" x 26" (outside measurement), and because they are tapered for nesting or making them stackable, the inside measurement is 16.5" x 24.5".

A commercial full sheet is 16" x 24". They are baked in 16" x 24" bakeable cardboard trays that fit into the Bun pans (flat surface portion) which are used during baking for support and handling purposes.

A true commercial full sheet (16" x 24") serves 96 (unit wt. 106-124 oz.).

http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm