You might get different answers but I usually double check how many servings are needed and then yes, use the 11x15. I don't trust the different definitions of "half sheet" and "full sheet" so I usually go by the Wilton party chart which is about 30 servings for an 11x15, and then just run that amount by the customer.
I consider that size "1/3" of sheet. My "1/2" sheet is 12x 18. I use the 11x15 all the time since it is great size. Most family parties have 30 people ballpark anyway. If it is bigger party they tend to want something fancier than 1/2 sheet.
I totally agree with Kitagrl!! I refuse to use the terms "half" and "quarter" because most consumers have no idea how big that is anyway. I'd almost bet you that if they asked you "how much is a half sheet?" and if you gave them a price ..... I swear, I'd almost bet their next question would be "Now, how many does that serve?" They use the lingo but they've no idea what they are asking.
When they ask for a price on a half sheet (or whatever), I do like kita said and ask them how many they are needing to serve. Then I tell them what size cake they need.
I checked one of my Wilton books and it says that it should serve 60. Does that mean filled? Also when I say filled is that torted and filled or two of them stacked??
Thank you so much for your respones. I already messaged her asking for how many servings.
The nice thing about square or rectangle cakes is that it's easy to just do the math to figure your servings.....
Single layer:
11x15 cake: 2x2" servings = cut columns/rows of 5x7 = 35 servings
11x15 cake: 2x3" servings = cut columns/rows of 5x5 = 25 servings
12x18 cake: 2x2" servings = cut columns/rows of 6x9 = 54 servings
12x18 cake: 2x3" servings = cut columns/rows of 6x6 = 36 servings
These are based on a single layer cake. If you bake two layers, then you will probably cut the pieces into 1x2" pieces to yield different serving amounts.
Double layer (pieces are 4 tall same as a wedding cake):
11x15 cake: 1x2 pieces = cut columns/rows of approx 10 x 7 = 70 servings
12x18 cake: 1x2 pieces = cut columns/rows of approx 12x 9 = 108 servings
Just do the math.....! 
Sizes, Servings & Prices
Sheet Cakes; Single Layer
9 x 13 (1/4 sheet) ; 24 ; $27.00
11 x 15 (1/3 sheet) ; 35 ; $33.50
12 x 18 (1/2 sheet) ; 54 ; $52.00
14 x 22 (2/3 sheet) ; 77 ; $74.50
18 x 24 (full sheet) ; 108 ; $101.50
That is for only one layer w/ no filling. It is more for 2 layers w/ filling.
I am also in the process of raising prices since my customers keep telling me they are too low!
I checked one of my Wilton books and it says that it should serve 60. Does that mean filled? Also when I say filled is that torted and filled or two of them stacked??
Thank you so much for your respones. I already messaged her asking for how many servings.
That is a double layer, I just divide that in half pretty much for a single layer sheet. Most people don't order a whole double layer 11x15.
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