I've been wondering this same thing now for a while and there have been several posts on this very topic. Some people say 12x18 is a 1/2 sheet, while others say its a 3/4 sheet. I couldn't find boxes to fit two 12x18 cakes so I just use 11x15 as a half sheet for now. Though if they need to feed more I would use the 2 12x18s. But I'm so confused as to what size is really the standard full sheet size. I think everywhere is a little different. I hope someone can help you more, and me too so that I know exactly what I should be using
There really IS no standard for this, because everyone seems to have a different opinion, Inluding bakeries. I personally, use an 11X15 for a half sheet, and two 11x15's for a full sheet. The only time I use my 12x18 is when the 11x15 isn't enough cake. Heiser17 is right, it is almost impossible to find a box that will fit two 12x18 cakes.
There really IS no standard for this, because everyone seems to have a different opinion, Inluding bakeries. I personally, use an 11X15 for a half sheet, and two 11x15's for a full sheet. The only time I use my 12x18 is when the 11x15 isn't enough cake. Heiser73 is right, it is almost impossible to find a box that will fit two 12x18 cakes.
there are three different options for 1/4, 1/2 and full sheets
7x11
11x15
15x22
8x12
12x16
16x24
9x13
12x18
18x24
I use the last grouping. And I always explain to customers that each bakery often use different sizes and that they need to be comparing servings to be sure they are talking about the same thing.
At the local grocery store I work at here in North Carolina I took my measuring tape to work today and measured our cakes, lol 
1/4 - 11 x 7-1/2 serves 24 cut in 2x2 squares $13.99
1/2 - 11 x 15 serves 48 cut in 2x2 squares $21.99
Full - 22 x 15 serves 96 cut in 2x2 squares (I cant remember this price I will check tomorrow and update my post)
I think in terms of volume. At the end of the day, though, all that really matters is that you are clearly communicating to your customer how much cake and how many servings they are getting for the price they are paying you.
But don't scare away business if your 1/2 sheet is larger than a local store sells - use it as an opportunity to bolster your argument for why your price is higher.
Personally I go the 1/4 = 9x13 route, since that's the pan I have (kitchen aid brand, if it matters). two of those = 1/2 sheet (or 18x13 when put together) and 4 = full sheet (18x26) (all sizes before any required trimming, so actual cake is a tad smaller, though the icing adds back a little, too).
One thing I try to discuss with someone is what THEY consider a serving vs. what a serving size really should be. IF they want a serving that is 4" square, they have to buy a much larger cake than I would have recommended (at a higher price, obviously).
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