9x13 or 8x12 are considered 1/4 sheet cake, even a 7x11 (each baker has it's own).
When I got my first 1/2 a sheet cake order i asked the same question here at CC and here is the link to that thread....
http://www.cakecentral.com/cake-decorating-ftopicp-316878-.html#316878
if you are using a 12x18 as your 1/2 sheet, I'd expect a 1/4 x sheet to be half that size (i.e., 9x12 - so I'd use a 9x13 pan)?
I do think there is some disagreement, but to me it is only logical that no matter what you use that you have 1 sheet = 2 half sheets = 4 1/4 sheets.
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