I have a baby shower cake that I need to make to fee 70+ people. The pans are square. The only thing I am having trouble with is what sizes should I use. It's a 3 tier cake. I'm attaching a picture of what the customer requested.

I was thinking 12", 8", 6" each pan is 2 " deep??? If anyone can guide me to what size cake pans I should use. Thanks!!

A 12" square by itself serves 72.
http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm