Wiltons cake chart says that a 2 layer cake that is in an 8" pan will serve 25 people
Also,my 2010 wilton year book says that stand is 16" 12" and 8" plates
have you looked in the Sketches & Patterns & Templates section of CC galleries? they have several that you can look at and print if needed for round and square cakes.
You can also try this website to help you determine what size cake you will need for X amount of servings. There was another thread about this and someone posted this info (sorry I don't recall who it was). It's been helpful to me.