Could anyone give me any suggestions on what pan size and number of tiers to use to make a beer can cake that will feed 40 people for a birthday party? Thank you for any suggestions and ideas.

Four 6" layers (or two 6" tiers) will yield 48 servings:

http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm

HTH