Leah_s numbers are the ones I am familiar with.
I'd go by how many servings they needed, not the size myself.
Sheet cake sizes (history of) and how to compute number of servings:
http://cakecentral.com/cake-decorating-ftopict-661804-math.html
Strange that everyone considers different sizing for this!
Who is this "everyone" that considers different sizes? Just curious. The industry standard sizes, as listed above by Leah_S are just that, industry standards, so there shouldn't be any variation, at least not among bakers. John Q Public, on the other hand, is generally clueless about most things when it comes to cake. 
prterrell, you wouldn't believe ....
I recall a thread in which a CC'er said, "The biggest pan I can fit in my oven is the 11x15, so I call that a full sheet." Hello!!!!!? Someone is making up terms based on how big their oven is? 
(although let me conceed that I can't remember now if the CC'er called it a full sheet or a half sheet ... in either case, she was wrong in the size and wrong in her logic. I think I have the thread saved on my laptop, but I'm goofing off at work right now.)
Grocery stores are starting to stock 11x15's and calling them half sheets as part of the new trend to reduce the size rather than raise the price (a half gallon of ice cream is now 1.5 quarts).
Agree on the John Q. Public thing, as illustrated by my friend who asked for pricing on a full sheet. When I asked how many guests he was planning, he said TEN! 
So I asked him, "So how come you want a cake that serves 100, then?
prterrell, I asked several bakeries around town and they all used a size other than 18x24 as a full sheet.
HEre's a supplier who is selling "standard" baking pans of 18x24: http://www.foodservicedirect.com/product.cfm/p/68102/Standard-Aluminum-Baking-Pan-Sheet-Size-18-x-26-x-1-inch.htm
Here's a link that pretty much gives a history of standard baking sizes, comparing the difference between commercial sizes and home-hobby sizes: http://www.makeacakehouston.com/id27.htm
Out of curiosity, what sizes did they give you as a full sheet? I'm wondering if they are using the terms as marketing to the general public instead of the true dimensions, such as "not many people order a full sheet to feed 100, so let's call the 50-serving the full sheet just so John Q. Public can use the terminology". (similar to the 1.5 quart "half" gallon of ice cream).
Just curious......
One was like PiePie's and 16", another was 15" - don't remember the rest - all close to 18 x 24, but not the "standard" 18x24. Even the packaging that we used (which was said to hold a full sheet) to box up the cake wouldn't hold our 18x24 and we had to trim the cake down.
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