What are the best size cakes for a two tier square wedding cake?

It really depends on how many people you need to feed. You can look up servings for cake sizes here: http://www.wilton.com/cakes/tiered-cakes/serving-amounts-all.cfm

If you want the cake to look bigger without adding a lot more servings than you need, you can also display it on a cake plateau, which you can probably rent from a party rental place or a cake shop, depending on where you are.

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