If 12x18x2 is a 1/2 sheet cake, then what is a 11x15x2 considered to be?

Some call it a 1/3rd sheet. But you are better off dropping the 'half' or 'quarter' sheet part and refering to them by size and servings. Most people have no idea how big or small sheet cakes are. Even bakers can't even agree on the termology of the various size sheet cakes.

Thank you so much TexasSugar... I will do that.

You are welcome. Here are a couple of past posts that talk about sheet cake sizing...

http://cakecentral.com/cake-decorating-ftopict-661804-size.html (love Indy's response.)

http://cakecentral.com/cake-decorating-ftopict-683297-size.html (defines sheet cakes)