Reganlynn it depends on the cake; what the event is; what the customer wants and how many servings needed.
If you are talking tiered cakes most are 4" tall ie: consisting of two 2" layers w/at least one layer of filling.
As for most b'day cakes I go by how many servings the customer needs. For less than 10 servings you could use a single layer 8" round or a dbl layer 6" round.
A sheet cake is almost always one 2" layer of cake. Can be split & filled if you want.
A wedding 'kitchen cake' usually is 4" tall w/as many layers of filling as the ',main' wedding cake so people think that's what they are getting. Again, that can vary depending on the wishes of the customer and how many servings needed.