If someone says they are having a party with say, 12 guests, how would I know how much cake to make? What I mean by that is, how many people would a 6" cake feed? 8"?

Go to Wilton.com and download a chart from their site. Good reference.

Here is the wilton chart, do note that these servings are for a 3"-4" tall cake using the standard serving size of 1x2x4
http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm

that's perfect, thank you so much! I should have known someone would have documented it