I'm doing a 3 tiered cake that needs to serve 100. The top tier is going to be a half ball that I will carve myself. I'm not sure how serving to count that top tier. Which would be the best sizing combination for this type of cake.
I guess it all depends what shape the second and third tier cakes are going to be. Also, I believe a regular 6 inch cake will serve around 10-12 people so if you are carving away at this then it would serve less. Hope that makes sense.
For what you are describing, I would probably use a 14" and 10" round as my bottom 2 tiers.