Full sheet - I use 2 12x18 and put them together
1/2 sheet is 12x18
1/4 sheet is 9x13
I hope this helps
My Wilton half sheet pan is is 11x15. But two of them together for a full sheet fits into the full sheet boxes just fine. The discrepancy about the half sheet cakes is sort if irritating to me. Sometimes I wonder if I'm not cheating people. But it does make a huge cake just the same.
My local bakery supply store actually carries full sheet boards. You might check the phonebook and see if there any other places that have baking supplies, etc. Or - you might check paper/box supply places? Also, there may be some good online resources for cake boards and boxes. I'm sure a more knowledgeable CC'er will have some links for you soon!!!
I tape 2 cake boards together - on the back side with wide clear shipping tape. Then I will use 2 19x13 boxes to transport. I fold the boxes leaving the front side unfolded and slide together this way and the tape the sides together.
I really try to talk my customer into 2 1/2 sheets instead of a full sheet - it's easier to decorate and easier for them to transport. But if they really want a full sheet I will do it. I also tell them to make sure they have a big clean trunk or suv/mini van to transport because it's really big and heavy. Most will end up just getting 2 cakes. I also tell them that doing it this way you can use one for the centerpiece & cut the other one until you need the center piece one.
Full sheet - I use 2 12x18 and put them together
1/2 sheet is 12x18
1/4 sheet is 9x13
I hope this helps
I've seen:
18x24 = full
12x18 = 3/4 sheet
11x15 = half
9x13 = quarter
It makes sense that the 12x18 is considered a half sheet and that's what I tended to classify it as ..... if it's the half, then what's the 11x15?
I'm repeating this, but this is exactly why I refuse to use these terms. Someone asks me "how much is your half sheet cake?" and I ask them "how many people are you needing to feed?" I get answers that range from 15 to 70 because Joe Blow on the street has no idea what these are either .... they're just using "the lingo"! 
so if they only need it for $20 people what sixe do you usually suggest?
For 20 people, I suggest the 11x15. When cut into 2x3" pieces you get about 25 pcs.
That's one reason I like sheet cakes .... it's easy to do the math to figure the servings.
Thank you so much i have been trying to figure out a good size for the piece and what size pan to use. I even went all out and drew a diagram with different sizes on a piece of paper to make sure the size of the pieces wern't too small.
I have been told I over think things sometimes.
hehehe!!! I think that the 11x15 was invented for peeps with itty bitty ovens or something to that effect. You know the ol' story bout the lady who insisted on cutting off the end of the ham before you cooked it because her mom always did, because Her mom always did? The lady asked the granma, "why did you cut the end off the ham before you cooked it?" She answered, "because that's the only way I could fit it into the pan I had at the time."
So, I am thinkin that this 11x15 thing came about in the same way and some manufacturers granma is out there laughin her head off because of it!!!
I made my own cake board out of heavy cardboard doubled. I taped it together and covered the whole thing with contact paper. Each time I use it, I cover it with freezer paper. The customer can pull off the dirty freezer paper and give me back the board which is covered in contact paper. I also custom fitted a box which I got from a furniture shop to fit it and covered it also with matching contact paper. I have notes on each of the four sides of the box to return the box and board to me with my address and phone number on it. I also tell the customer I want the box and board back. I made two when I made them. I'm down to one. I hope I don't lose it.
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