Does anyone know the measurements of the sheet cakes sold in bakeries?
call your local bakery and ask them.
"Hi, I'm planning a baby shower. How big are your cakes? ...... I'm afraid I've no idea how big a "half" sheet cake is. Can you give me the measurements so I can get an idea of the size? Thanks!"
Don't be surprised if you run into someone who will never make more than minimum wage their entire life. I got one of those once when I was doing a price check and needed the exact size of the cake. Here's the story: http://forum.cakecentral.com/cake-decorating-ftopict-615808-ruler.html