I know Wilton.com has serving size charts. Here is one chart. I know there is another one for cutting the cakes as well. I basically go by what wilton says, probably because most of my pans are Wilton. I hope this helps!
http://www.wilton.com/wedding/cakeinfo/choosing/all.cfm
I think the Wilton chart is for cuts that are really too small. Only a very skilled cutter can accomplish those tiny pieces. There is also a Party cut size chart that allows for slightly more realistic cuts.
And, Earlene has a chart for sizes that is for somewhat larger cuts.
http://www.wilton.com/wedding/cakeinfo/partycuttingguide.cfm
This link should get you to the party cuts guide.
But when you make a cake for, say, a birthday party, they're going to cut the pieces bigger, right? I think the guide I have says that a 9in layer cake should serve 32 people. I don't know anyone who would even want to cut the pieces that small. If I made a cake like that and then told a customer it served 32 I imagine she would think I had lost my mind! LOL
I figure 10 servings per cake mix used. I know this is large, but I add filling and people always want more. If I do a character cake I do a base 1/2 sheet using two mixes and the character 1 mix. I tell them it feeds 30. If they want to feed 50 I stack 3 2-mix half sheets. I believe it is always better to have too much than too little. Charge accordingly. The aforementioned 50 person cake I would charge $50-$60 for it depending on the design, etc. Wilton pieces are too small in my opinion also. If I am cutting the cake at a party I use a 2"x2" square to start and let then let the customer cut any seconds that people want.
Quote by @%username% on %date%
%body%