I thought an 11x15in sheet pan would feed 35 but now I'm scared it won't! Maybe I should make it 2 layers? I don't have the next size up pan and I have a cake due on Friday for a customer.

http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm

I use the above for all cakes I do. It is for two layers, so according to it, a two layer 11x15 would feed 74 1x2 in servings. If you divided that in half, it would give you about 37 pieces.

Interesting article about sheet cakes and how many they feed. Also has a little diagram.

http://www.hurryupcakes.com/general-cake-information/how-many-people-does-a-sheet-cake-feed

I was just told by a bakery that a "serving" is 2" x 2" and she told me to calculate like this: Say you have a 10 x 14 pan. The 10 in. side will give 5 two inch pieces and the 14 in. side will give 7, so 5 X 7 = 35. So your pan is a little bigger than that, so it would yield a little more than 35 2 x 2 servings. I hope that makes sense!