here is a link from wilton that you might find helpful. Scroll down on the page and there will be a chart with pan sizes and serving amounts.
http://www.wilton.com/cake/cakeprep/baking/times/index.cfm
here's another one from Earlene's
http://www.earlenescakes.com/ckserchart.htm
These charts are just guides though, because, like Traci said, it all depends on how large they cut their pieces.
I do the 11x15 pan as two layers of cake and one layer of filling and tell my customers that it will feed approximately 35 people. I personally have gotten 40 servings from this cake size (filled).
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