I was asked about making a sheet cake for a lady's 70th birthday to feed roughly 60 people. I have never made one that size and don't know what to charge or what size to make it.
You should charge by the slice, call around your area to see what the going rate for bc sheet cakes is. Don't call walmart or costco call bakeries big box stores are a different animal all together!
An 11x15" should do or a 12x18
Use a 12x18...that is what wilton suggests for 60 pieces
An 11x15, when cut in approx 2x2x2 pieces (single layer) will serve 35.
An 11x15, when cut in approx 1x2x4 pieces (double layer) will serve 70.
A 12x18, when cut in approx 2x2x2 pieces (single layer) will serve 54.
A 12x18, when cut in approx 1x2x4 pieces (double layer) will serve 108.
Be sure to charge for the number of servings the cake is DESIGNED to serve ... not the number of pieces they plan to cut from it.