What size pan is considered a 1/2 sheet cake ?
A half sheet can mean different things to different people! If you're doing this for someone, ask them what they mean; also ask them how many servings they need, that is more important.
In general though, I think that a majority of people mean 12x18 when they are talking about a half sheet.
What she said ^^^. I know so many people who think that a sheet cake just means a rectangular cake, like the ones at the grocery store. Silly non-cakers!