Here is Wilton's guide, which has serving amounts based on wedding-sized portions of approximately 1 x 2 in:
http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm
In addition to the above chart (which you should print out and keep handy), here is a link to show you how to cut a cake to achieve those servings: http://www.cateritsimple.com/id10.html
Here is a photo of 1x2x4 pieces of cake, so you can see they are not teeny-tiny, but are a nice dessert size serving: http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1156785
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