History of cake pan sizes:
A commercial baking sheet pan is 18x26 (or sometimes 18x24)...full sheet.
Cut that in half and you now have two 12x18's.....half sheet.
Cut those in half and you now have four 12x9's....quarter sheet.
I think Wilton threw the 11x15 in there just to screw us up!
Here is a link to a thread where I explain how to "do the math" to figure how many servings you get for each size cake, based on single layer cakes: http://cakecentral.com/cake-decorating-ftopict-650914-math.html