For wedding cakes you should be ordering based upon 'finger' slices because cake is usually served with tea and coffee following a dessert course. However if you will be using the cake as your dessert you should allow for a 'dessert serve'.
For birthday or celebration cakes it depends if you will be having other desserts or cupcakes. If there are other sweet treats available then you will only need a 'finger' slice for your guests. If the cake is your dessert you can go with 'dessert serves' to provide an adequate slice. For single tier cakes we have also included a 'party' size, which is perfect if your guests require a big chunk of cake or you want some leftover to have the next day/give some for everyone to take home.
Next you have to choose between the Standard 6" height or the Everest 8" height per tier. Both are very elegant looking, but the taller size has a bit more wow factor if that's what you're after! The Everest also provides your guest with a larger portion of cake - 4" tall instead of 3". The photo above shows the difference between the two.