Sizes for a half sheet cake vary from person to person, baker to baker. My half sheet is 11X15 because that's the biggest size of pan that will fit into my oven. I charge by the serving so my "1/2 sheet" serves 30 and I price accordingly. If my oven were a tiny bit bigger, I could use my 12X18 and that would then be my 1/2 sheet cake! It's up to you!!! You just need to know how many servings the customer can expect to get from your "1/2 sheet." (IMHO)
"half sheet" is a name and it means different things to different people. Its easier to go by number of servings. If you walk into a supply store and ask for a full sheet pan, they will hand you a 24x18. It then follows that a half sheet is 12x18 and quarter is 12x9. These are the sizes I use.
This what i found from an earlier post i hope it answer all questions
1/2 sheet to be 12 x 18 and you can get 40 - 48 pieces.
Full sheet pan Standard 24x16x2
3/4 sheet pan- 18X12X2
Half sheet pan Standard 16x12x2
Quarter sheet pan Standard 13x9
Quarter Sheets, Half Sheets, and Full Sheets Explained
The retail baking industry developed standard sizes for rectangular sheet cakes, beginning with their standard sheet pan size of 16â by 24â and dividing it into halves (half-sheet) or quarters (quarter-sheet). The manufacturers who supply paper and plastic items to the baking industry use these standard sizes to create a variety of cake boards, boxes, and domes.
In an effort to develop a unique market share, some manufacturers began promoting pans of slightly different sizes, selling mainly to home bakers through hobby and specialty stores. Since home bakers do not have the volume purchasing power of retail bakers, most of the paper and plastics manufacturers have not created boards or boxes to accommodate these sizes.
The bottom line: Boards and containers for sheet cakes will always be easier to find if you use the retail baking industry's standard size pans. You will save yourself a lot of frustration by sticking with the standards.
Baking and Paper Industry Standards; The sizes below are recognized as industry standards.
Quarter Sheet Cakes: 8 x 12 pan, 10 x 14 cardboard
Half Sheet Cakes: 12 x 16 pan, 13.5 to 14 x 18.5 cardboard
Full Sheet Cakes: 16 x 24 pan, 19 x 27 cardboard
Non-standard Sizes. These sizes are not standard sizes. Suitable boards and containers can be very difficult to find.
9 x 13 pan: We call it the âlarge quarterâ size. Very few of the paper suppliers carry the boards (11 x 15) and boxes (12 x 16) for cakes made in 9 x 13 pans.
10 x 15 pan: Use the standard half sheet boards and boxes
11 x 15 pan: Use the standard half sheet boards and boxes
12 x 18 pan: We call it the âlarge halfâ size. This size pan will not fit in most home ovens. Wilton sells a set of 14 x 20 boards that can be used with this pan, but they sell no boxes. If you cannot locate a âlarge halfâ board (14 x 20) and box (14.5 x 20.5), you will have to use full sheet boards and boxes.
This whole thread is why I just refuse to use those terms. People who "just decided" that an 11x15 is a half sheet because that's the biggest pan that will fit in their oven "..... so that must mean it's a half sheet." what???? 
If you had a tiny oven, would you call a 9x13 a half sheet just because that was the biggest pan that would fit? I really don't understand the logic here.
If my garage is only big enough for a Volkswagon, that doesn't mean I can call it a Cadillac Escalade SUV. It's still a Volkswagon.
No wonder cake civilians have no idea what the heck they're talking about when they loosely throw around terms like 'half' and 'quarter' sheet. When cake makers ..... whether they be professional or hobbyist ... just make up a dimension for the term based on the size of their oven, , then how would we expect non-cake-people to understand it?
And it's why, when people ask for a half or full sheet, I just ask them how many people they need to feed. I will not use the terms for just these reasons listed on this thread..... the fact that too many people have no idea what they're talking about.
And for the record, everything I've ever read indicates an 18x24 (ish) is a full sheet, so the 12x18 is a half sheet. No matter how big your oven is.
......The bottom line: Boards and containers for sheet cakes will always be easier to find if you use the retail baking industry's standard size pans. You will save yourself a lot of frustration by sticking with the standards....
Basically this is what I have b een trying to get across to people for years now!
*Match your pans to the size board & box being sold*
Which to me means a "1/2 sheet" is 12x16 so y ou have room for a border.
When someone orders a '1/2 sheet' ask them how many servings they need. Then tell them "an 11xc15 will give you X# of servings".
I've had people ask for a full sheet when they only wanted like 25 servings.
I only use the terms 1/2 sheet cake when customers start their cake requests with, "I need a 1/2 sheet cake"---I then lead them into the conversation by saying, "My 1/2 sheet cake serves 30"--and we progress from there." Those poor cake civilians have to start somewhere and some days I feel like I'm finally putting my elementary education degree to good use 
!!
"half sheet" is a name and it means different things to different people. Its easier to go by number of servings. If you walk into a supply store and ask for a full sheet pan, they will hand you a 24x18. It then follows that a half sheet is 12x18 and quarter is 12x9. These are the sizes I use.
They said it best! 
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