Many custom bakeries in the USA use the Wilton Wedding serving chart to determine the pricing per serving. Individual custom bakers may choose whatever they want to use to price their cakes. Keep in mind that NOBODY out there in the general public has any idea how to get 24 wedding cake slices out of an 8" round, 4" high cake tier. [They'll think you're NUTS!] Many wedding cakes are cut and served by a professional caterer or staff person at the venue. The assumption with wedding cakes is that there will be other food served at the event, and the cake will be a symbolic, small slice. Party or Earlene sized slices are generally used when there is no other food at the party/celebration.
Here is a lovely chart from the Lark Cake Shop:
Indydebi has another wonderful method which you can copy and provide to clients. Indydebi states:
"feel free to link to this page or print it out for your brides to help them ensure a smooth cake cutting at their event!"
These sample cake serving slices are also excellent to give to a client. The client can see how big the slice will be on the cake AND know how big the slices should be when cut!
Sample Cake Serving Sizes -- made of Paper or Cardstock