I refuse to use the term "Half" and "Full" sheet and here's why:
http://forum.cakecentral.com/modules.php?name=Forums&file=viewtopic&p=3245473#3245473
In my neck of the woods:
1/4 sheet - 9 x 13
1/2 sheet - 11 x 15
whole sheet - 12 x 18
For my purposes:single layer cake...............two layer cake
1/4 sheet = 9x13 = 24 (2x2) servings........48 (1x2x4) servings
1/2 sheet = 12x18 = 54 (2x2) servings......108 (1x2x4) servings
Full sheet = 18x14 = 108 (2x2) servings....216 (1x2x4)servings
My clients understand what my terms mean.
Choose whatever works for you.
While I tend to agree with Debi, sometimes for quick reference you do want to 'label' the sizes. In that case here is what I have found to be MOST standard. Not everyone uses this, but it's the most common
9"x13" - 1/4 Sheet
11"x15" - 1/3 Sheet (roughly)
12""x18" - 1/2 Sheet
18"x 24" - Full sheet (this is simply 2 half sheets side by side.)
With absolutely NO disrespect intended to cakeladydi, 12"x18" cannot be considered a full sheet if 11"x15" is a half. You'd have to do two 11"x15"'s side by side (resulting in a 15"x22") for a full sheet. Only because of the sheer math of the situation. Again, I absolutely do not mean to say you are wrong cakeladydi, simply differing info! Also, size will differ by area... some people don't use the 11"x15" at all, (I don't currently use the 12"x18" until I redo my kitchen because I have a godforsaken small oven!!) And some DO use the 11"x15" and consider a full sheet to be two of those, and do not use the 12"x18". Have I confused you enough yet? 
Again, this is all just the most common info I've gathered from reading a LOT of threads on this topic! Hope it helps a little!
...12x18. When I went to the cake store to buy the pan the had it labeled as a half and hey had 11x15 labeled as a half...
Yah, they're in no hurry to make it easy on us are they? hahaha! How are we supposed to know when THEY don't even know! 
This is a sore subject for me when quoting prices to my customers!
Commercial bakeries (at least here) use smaller measurements for their sheet cakes. That comes from the fact that they start out with a full sheet pan, then cut the size they need. That pan won't fit into a domestic oven, so I'm never going to get those sizes. So while I may call them the same thing as commercial bakeries, my cakes are larger and I just have to suffer with that until I get that coveted professional oven.
At least here, these are the numbers the big guys use:
8x12-1/4 sheet
12x16 - 1/2 sheet
16x24 - Full sheet
But for the home bakers purposes, I agree with Melvira's numbers.
Here, prices are not set by the serving unless it's a wedding cake, so whether I personally like it or not, I use what the big boys use and that's sheet sizes, not number of servings. I'm not out to change the world, just bake a cake.
Hey justme50, I am glad to see the info you provided because I have often wondered about that. I have been wonderin if you can find an 8"x12" pan somewhere because the commercial 1/4 sheet pads and boxes I get are only 10"x14" so if you put a 9"x13" in there, your borders either fall off the board or get smooshed in the box! Does anyone know where you can find an 8"x12"?
The closest I've come is just the cheapie aluminum foil pans. I found some at a Dollar General store that measure 12 1/4 x 8 1/4 x 1 3/32. They were going out of business so I bought a bunch. I'm not fond of using them, but I'd like to start using bakery standard sizes.
My husband works for a wholesale bakery so I managed to get hold of a professional full sheet pan. 4 of these aluminum pans fit it exactly.
It's amazing the difference in size and depth of what most home bakers use compared to the cake size you'll get from a "regular" bakery. In these pans, I can't get a whole cake mix in them, usually have enough left over for several cupcakes or a 6" round. If I doctor the mix, it's even more leftover.
My customers are used to the smaller bakery size and don't allow for the fact that my cakes are actually larger. I'm really just wasting money when I give them the larger home baked version.
Melvira: No offense taken here. I completely understand where you are coming from. Most of my customers don't ask for 1/4 or 1/2 they just say, "I need that big sheet cake" (I know they mean 11x15) or "I just need a small sheet cake" (9x13). They usually know if they need something bigger that the 11x15 so then we discuss servings.
I hardly ever do the 12x18. I would rather do 2 9x13 and put them together.
I really appreciate all the helpful info provided here on CC and the sweet spirit with which it is given. Thanks Melvira.
Hey justme50, I am glad to see the info you provided because I have often wondered about that. I have been wonderin if you can find an 8"x12" pan somewhere because the commercial 1/4 sheet pads and boxes I get are only 10"x14" so if you put a 9"x13" in there, your borders either fall off the board or get smooshed in the box! Does anyone know where you can find an 8"x12"?
I hate that too especially since I like to have a good 1"-2" of board on all sides. Since I don't really sell much it isn't a problem. I'm making a 9x13 for my nephew's birthday and traveling with it so I bought a half sheet box so I'd have plenty of room for borders without worrying about squishing.
yeah, I agree with the whole confusion thingee. We bake half sheet (12x1
and then cut them in half for the quarters and lay two cakes long sides together for full sheets. Well, at least that's how it's done at our bakery. Also want to point out too that, that is also how they do it at grocery store bakeries too. (well most of em anyways).
This sheet cake confusion kinda reminds me of tryin to buy ink for your printers, sheesh now THAT is confusing.
I use the same exact sizes as Melvira, but like Indydebi don't get into any discussions with the customer over what each size is called. I find out how many servings are needed and then use the pan that best accomodates the servings.
Melvira, there are a few places you can find a 12" x 8" on the internet. Here is a link for one of them, but I've never ordered from these people, so I can't say whether they're reliable or not.
http://www.bakingtools.com/productdisplay.aspx?catid=83&c=Sheet_Cake_Pans
I use the same exact sizes as Melvira, but like Indydebi don't get into any discussions with the customer over what each size is called. I find out how many servings are needed and then use the pan that best accomodates the servings.
Ditto that. I always end up telling them what they need. lol
Hey justme50, I am glad to see the info you provided because I have often wondered about that. I have been wonderin if you can find an 8"x12" pan somewhere because the commercial 1/4 sheet pads and boxes I get are only 10"x14" so if you put a 9"x13" in there, your borders either fall off the board or get smooshed in the box! Does anyone know where you can find an 8"x12"?
Just thought I'd give this a bump because I finally found some decent 8x12 pans!! They now have them at intotheoven. The brand is Fat Daddio and as I understand it, that's a pretty decent quality pan.
Also, if you go to their forum and check out the posts, you'll find a 20% off code...good only through tonight though!
I didn't submit the order last night because I'm hoping they'll have another one. Anyone know anything about that??? Melvira I found a 8 x 12 here on this site...just ordered it along with a few other things 
http://cooksdream.com/store/sheet-cake-3-fat-daddios.html
Sorry you had trouble finding it. They ask us not to give too much information out because they really want to encourage people to start using the forums.
They usually don't hide them where it's really too hard to find them and post them sort of where the item they're discounting belongs. In this case, since it was for a cake pan, it was under the cake forums, in the general category.
If you'll register and post there with some regularity you stand a good chance of getting coupon codes emailed to you too. I post there a few times a week and have already had 3 codes mailed to me.
But yes, they post discounts quite often...they hardly ever go a week without posting one.
I ordered the 8x12 along with a few other items and my shipping was only $6.50...you just can't beat their prices, discounts or not!
oh I found this website in another post and checked out their pans and they have all sorts of them including the odd sizes you are looking for.
HTH
http://www.wineandcake.com/browse.cfm/2,106.html
When people say 'i want a sheet/half sheet/whatever' i want to roll my eyes....but i don't....i just say, 'how many people were you hoping to serve?' and then take it from there....
i obviously get confused as well....
i also don't know whether a 'sheet' of anything is considered to be just one layer.....or two....or one, torted and filled....
that's why i do rounds/squares!!!
glad to see i'm in good company....
Sally
Your welcome Melvira
justme when I clicked on there (to see about getting one myself) it said they were sold out
Well darn! I just looked too and they don't have the 8 x 12 listed now. They do have a 7 x 11, but that's a bit too small for me.
Hopefully they'll restock, but those other links you provided have some pretty decent prices too.
When people say 'i want a sheet/half sheet/whatever' i want to roll my eyes....but i don't....i just say, 'how many people were you hoping to serve?' and then take it from there....
i obviously get confused as well....
i also don't know whether a 'sheet' of anything is considered to be just one layer.....or two....or one, torted and filled....
that's why i do rounds/squares!!!
glad to see i'm in good company....
Sally
Unfortunately, most of my customers have been "walmartized" into talking sheet sizes when they really have no idea what a specific sheet size will serve. No one, outside of wedding cakes, seems to want round cakes here.
So, I just have standardized sizes and layers that I use as a starting point. For me, a standard sheet cake is a single layer, no torting, no filling, just like WalMart. Anything beyond that point and I start adding additional charges. I use the standardized sizes of 8 x12, 12x16 and 24x16 for my 1/4, half and full sheets. I always ask how many they want to serve to be sure the sheet size they're asking for will work.
I ordered the 7 x 11 from the site I gave Melvira and got it in a day , I was very pleased. I don't know what exactly I am going to do with it because it is a tad small for a 1/4 BUT I figured that I could charge 5 dollars less for it then the 9 x 13 AND if someone is having a smaller party there won't be that much cake left
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