This diagram shows a 9 x 13 cake cut into 21 pieces. Each serving measures 1.8 x 3. While those are the standard sizes, there is only one way to be 100% how big the “half sheet cake” your baker is making is: ask.
http://www.hurryupcakes.com/general-cake-information/how-many-people-does-a-sheet-cake-feed
I would tell someone my half sheet is $75 and they would tell me the grocery's 1/2 sheet is $50 and I'd have to explain to them that the grocery's half sheet is NOT a half sheet and its not as big as my LEGIT half sheet.
http://www.cakecentral.com/forum/t/715491/sheet-cake-sizes
This is what I found when I did a search for your answer. The rectangular pan that most home bakers are familiar with (and that 1 box of Duncan Hines, Betty Crocker or Pillsbury cake mix will make one of) is the quarter sheet pan, which measures 13x9, with a depth of either 2 or 3 inches. If you laid 4 of these pans together, this would be approx. the size of a full sheet.
http://www.cakecentral.com/forum/t/715491/sheet-cake-sizes
A sheet is typically one layer that you get from the grocer. If you flip your pan over you can measure how many serving you can get from that pan using 2 for your sizes. So yes, your 11x15 will make 30 servings one layer or 60 for two.
http://www.cakecentral.com/forum/t/715491/sheet-cake-sizes
Most bakeries in the U.S. have three standard sizes of sheet cakes available. 1  1/4 Sheet Cake, 9″ x 12″, serves 12-28. 2  1/2 Sheet Cake, 11″ x 15″, serves 18-40.  Full Sheet Cake, 18″ x 24″, serves 48-108.
http://www.hurryupcakes.com/general-cake-information/how-many-people-does-a-sheet-cake-feed
Cotsco half sheet cakes are approximetaly 8 inches by 11 inches, which is the size of a normal sheet of paper that you can buy at the stores, and the size of an 8x11 picture. Cotsco half sheet cakes are approximetaly 8 inches by 11 inches, which is the size of a normal sheet of paper that you can buy at the stores, and the size of an 8x11 picture.
http://www.answers.com/Q/How_big_is_a_Costco_half_sheet_cake
This diagram shows a 9 x 13 cake cut into 18 pieces. Each serving measures 1.8 x 3. It’s Party Time and you’ve decided you’d like to serve a sheet cake, but there’s only one problem: What size sheet cake has the right number of servings for the people you wish to feed.
http://www.hurryupcakes.com/general-cake-information/how-many-people-does-a-sheet-cake-feed
If you’re planning to bake a sheet cake at home, the most common size cake pan is 9″ x 13″ (slightly larger than the one-quarter sheet cake offered by bakeries) Homemade Sheet Cake, 9″ x 13″, serves 13-28.
http://www.hurryupcakes.com/general-cake-information/how-many-people-does-a-sheet-cake-feed
A cake mix can make approx. 4 -- 5 cups of cake batter, although it can vary …by 1/2 cup or more depending on how big your eggs are, as well as how long you mix the batter and at what speed. When I bake them, I make 3 boxes. Fill the pan 2/3 full. You'll have a little batter left over.
http://www.answers.com/Q/How_big_is_a_Costco_half_sheet_cake
In the U.S. there are four “standard” sizes of sheet cakes. Most bakeries in the U.S. have three standard sizes of sheet cakes available. 1  1/4 Sheet Cake, 9″ x 12″, serves 12-28.  1/2 Sheet Cake, 11″ x 15″, serves 18-40.
http://www.hurryupcakes.com/general-cake-information/how-many-people-does-a-sheet-cake-feed
how big is a normal sheet cake?
A normal sheet cake is 9 inch by 13 inch big.