I've always considered it to be 18x24...some others have different opinions but my cake boards that I buy for a "full sheet" fit the 18x24 just right! Since most regular ovens won't take that large of a pan you can easily do it with 2 12x18 set together.
Yep 
The standard is:
9x12 is a quarter sheet
12x18 a half sheet
18x24 a full sheet
I usuall use two half sheets attached if someone wants a full sheet cake.
My pans have perfect 90 degree sides so there is no gap. Besides.. I have found that most folks that want the look of a full sheet cake still want you to make it with two flavors.
Whatever. 
problem with the boards for that size is they aren't strong enough for a cake that size, at least mine weigh a ton. Go to Michael's or whatever craft store you have and buy foam poster board by Elmer's . It's really heavy duty and I only ever need one board to support the cake.
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